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Electrical 

Engineering Testing 


FROM THE 

DIRECTLY-USEFUL TECHNICAL SERIES 


ARITHMETIC FOR ENGINEERS 

INCLUDING SIMPLE ALGEBRA, MENSURATION, LOGARITHMS, GRAPHS, 
THE SLIDE RULE, VERNIERS AND MICROMETERS 

By CHARLES B. CLAPHAM 

Hons. B.Sc. Eng. (Lond.) 

Lecturer in Engineering and Elementary Mathematics at the 
University of London—Goldsmiths' College 

Demy 8vo. 477 pages, 167 figures, and over 2,000 worked 
and set examples with answers 


MATHEMATICS FOR ENGINEERS 

By W. N. ROSE 

B Sc. Eng. (Lond.) 

Late Lecturer in Engineering Mathematics at the University of 
London — Goldsmiths’ College. Teacher of Mathematics, 
Borough Polytechnic Institute 

The two volumes of Mathematics for Engineers form a most 
comprehensive and practical treatise on the subject, and will prove 
a valuable reference work embracing all the mathematics needed 
by engineers in their practice, and by students in all branches 
of engineering. 

PART I 

Demy 8vo. 510 pages, 257 figures with over 1,200 worked 
and set examples. 

PART II 

Demy 8vo. 436 pages, 142 figures and nearly 1,000 
worked and set examples 





The Directly-Useful 



Technical Series 


Founded by the late Wilfrid J. Lineham, B.Sc., M.Inst.C.E. 


Electrical 

Engineering Testin 



A PRACTICAL WORK 

ON CONTINUOUS AND ALTERNATING CURRENTS 
FOR SECOND AND THIRD YEAR STUDENTS AND ENGINEERS 


r-L 


BY 


Gf d/aspinall parr 

M.Sc. J M.Inst.E.E., A.C.G.I. 

Diplomee in Physics and Electrical Engineering 
of the Central Technical College, City and Guilds of London, 

Eormerlt) 

Head of the Electrical Engineering Department, The University, Leeds, 
Ckaimcn of the North Midland Section of the Jnst.E.E., 

Examiner in Electrical Engineering in the Universities of Leeds and Nenv Zealand, 
Technical Adviser to the Mexican Government, 

Associate Member of the Institute of Mechanical Engineers, 


FOURTH EDITION 
REFJSED AND ENLARGED 
» 

» > 

> ) > 

’ » ' 

NEW YORK 

E. P. DUTTON & COMPANY 

681 FIFTH AVENUE 

1922 


Monograph 





Printed in Great Britain by 
lliCHARD Clay & Sons, Limited, 

BUNGAY, SUl'TOLK. 


am 

PaWishe) 
SEP i4 ia2l 


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1 

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j 

^ EDITORIAL NOTE 

The Directly-Useful Technical Series requires a few 
words by way of introduction. Technical books of the past have 
arranged themselves largely under two sections: the Theoretical 
and the Practical. Theoretical books have been written more 
for the training of college students than for the supply of 
information to men in practice, and have been greatly filled with 
descriptions of an academic character. Practical books have 
often sought the other extreme, omitting the scientific basis 
upon which all good practice is built, whether discernible or not. 
The present series is intended to occupy a midway position. 
The information, the problems, and the exercises are to be of 
a directly-useful character, but must at the same time be wedded 
to that proper amount of scientific explanation which alone will 
satisfy the inquiring mind. We shall thus appeal to all tech¬ 
nical people throughout the land, either students or those in 
actual practice. 


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PREFACE 

This work is intended to form a systematic course of instruc¬ 
tion in the very extensive field of testing connected with pure 
Electrical Engineering. While much has been written, from 
time to time, about the more elementary branch of testing in 
Electrical Physics, relating in a measure to Electrical Engineer¬ 
ing, I believe that, so far, no extensive attempt has been made 
to treat the more advanced and practical portions of the subject 
in that systematic manner which it requires. 

I therefore venture to think that the present work, which not 
only embodies much of, if not all, the experimental work that it 
is usually possible to do at most colleges, but also many tests on 
heavier electrical machinery, together with a highly descriptive 
course on jointing Electric Light cables, should be eminently 
suitable for constituting the electrical laboratory practice in the 
second and third years of a complete course of instruction in 
Electrical Engineering, and in addition should be of consider¬ 
able service to the electrical engineer in electrical works and 
central stations. As far as possible the tests have been arranged 
in the order in which they may be worked, when used as a course 
for students, but there are exceptions to this rule, owing to the 
advisability of keeping tests of a similar nature together. I 
have endeavoured to make the tests as complete and descriptive 
as possible, and applicable in the ease of any college and testing 
room. Each test comprises—an Introduction giving the chief 
features, advantages, and disadvantages of the test, condensed as 
much as possible j the Apparatus necessary; the Observations to 
be carried out, in other words, a complete and carefully arranged 
digest of the method of carrying out the actual test, with a 
Diagram of Apparatus and connections represented symbolically; 
a Tabular Form indicating the most convenient and proper way 
of recording the observations; and finally, Inferences which cim 
be drawn from the results of the test. These latter if conscien- 


Vll 


VI11 


PBEFAOB 


tiously worked out are calculated to cause the experimenter to 
think and reason for himself. ^ 

Following the series of tests is an Appendix, containing the 
Algebraical solutions of the various formulae met with in the 
tests, and these the student is strongly urged not to refer to until 
he has tried, by all the means in his power, to solve the inference 
for himself. 

The Appendix also contains complete descriptions and illustra¬ 
tions of a large amount of the apparatus which may be employed 
in carrying out the tests, a considerable proportion of it being 
such as will be found in almost every college and testing-room. 

Useful constants and tables which are frequently needed in 
electrical engineering tests are added at the end of the book. 
A good deal of the apparatus illustrated has been constructed by 
the mechanical assistants, Messrs. John Watkinson and Herbert 
Addy, of the Electrical Engineering Department of the Univer¬ 
sity, Leeds. 

In conclusion, I wish to express my sincere thanks to my 
valued friend, Mr. Charles Mercer, M.A., for the very consider¬ 
able amount of trouble he has taken in producing the photo¬ 
graphs from which many of the illustrations are obtained; 
to Dr. John Henderson, for permission to use the tables of squares 
and reciprocals of numbers; to Messrs. Kelvin & James White, 
for permission to use the tables of doubled square roots; to His 
Majesty’s Stationery OflSce, for allowing me to use the tables of 
logarithms and anti-logarithms; to Messrs. Longmans, Green & 
Co., for their kindness in lending eight illustrations and a 
little printed matter from Practical Electrical Testing; to Mr. 
Herbert Addy, for the trouble he has taken in making the 
drawings from which the illustrations of joints, made in electric 
light cables, are taken, and also for reading through the proofs 
simultaneously with myself; and further to Messrs. Nalder 
Bros. & Co., Kelvin & James White, Siemens Bros. <fe Co., 
Crompton <fe Co., and Evershed h Vignoles, for their kindness 
in lending me the blocks of some of the illustrations of the very 
excellent apparatus and appliances made by them. 

G. D. A. P. 

The University, Leeds. 


I 


J 


PREFACE TO THE FOURTH EDITION 

While opportunity has been taken, in previous editions, to 
both enlarge and improve the book, the scope of it has under¬ 
gone very considerable extension and rearrangement in the 
present edition. Of some 132 extra pages of new matter, no 
less than 116 represent entirely new tests, including a little 
additional theoretical explanatory matter to the previously 
existing tests, while the remainder comprise descriptive matter 
and tables of useful figures. 

Some of the new tests are of a direct current nature and brinjr 
the direct current portion of the book more thoroughly up-to- 
date, but the remainder, forming by far the greater proportion 
of new tests, relate to alternating currents. This branch of 
practical work—always more difficult to understand than that of 
direct currents—has therefore been greatly strengthened by 
additional matter dealing with modern theory, laboratory and 
commercial tests supplemented by vector diagrams which enable 
the phase relations between current and pressure to be more 
easily understood. 

Further, the greater ^portion of the work has been completely 
rearranged so as to have all tests of a like nature together, 
though not necessarily numbered in the order in which they 
should be taken. The author therefore hopes that this edition 
will be found to offer many advantages over previous ones, and 
he will welcome notification of any errors which may have 
escaped observation before going to press. 

I would like to thank my friends who have so kindly helped 
me to read through the proofs of this edition simultaneously 
with myself; also Messrs. Evershed & Vignoles, Nalder Bros. & 
Co., and Elliott Bros. Ltd., for their kindness in lending me 
additional blocks of illustrations of apparatus; and Messrs. 
The London Electric Wire Co., Smiths Ltd., for permission 
to reprint their Tables of Resistance Wires. 


ix 


January 1922, 


G. D. A. P. 




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CONTENTS 


Curve Plotting. 

Calibration and standardization of Ammeters . 

Calibration and standardization of Voltmeters 

Complete Test of D.C. and A.C. Ammeters and Voltmeters 
for all Sources of Error . . • . 

Calibration and standardization of Wattmeters 

Calibration and other tests on Electricity Meters 

Efficiency and other Photometric tests on Electric Glow 
Lamps ........ 

Efficiency and other Photometric tests on Electric Arc Lamps 

Efficiency and other tests on Secondary Cells . 

Measurement and comparison of High, Ordinary, and Low 
Metallic Resistances ...... 

Measurement of the Insulation Resistance of Cables, Cells, 
Insulators, Machines, and Installations . 

Localization of Faults in Electric Light Mains 

Measurement of Rotational Speed by Speed Counter and 
Watch, and also Stroboscopically .... 

Determination of the “Characteristics’’ of Direct Current 
- Dynamos ........ 

Determination of Field Winding and Speed Conditions for 
True Compounding of D.C. machines 

Determination of the Characteristics, Regulation, and 
Efficiency of Alternators . . . 

Determination of Potential around the Commutators of 
Dynamos and Motors ...... 

Separation of Losses, Leakage Coefficient, Effect of Air- 
gap, Faults in Field Coils, Temperature Rise, in 
Dynamos and Motors ...... 

Efficiency and regulation of Direct Current Dynamos 


PAG* 

1 

4 

16 

29 

34 

43 

50 

61 

75 

81 

97 

130 

134 

140 

165 

168 

195 


202 

219 


XI 


xii CONTENTS 

Efficiency, and regulation of Direct Current Motors 

Efficiency, and regulation of Alternating Current 

Induction Motors ...... 

Efficiency, B.H.P., Begulation, and other Characteristics 
of A.O. Commutator Motors .... 

Efficiency, B.H.P., Begulation, “V” Curves, and other 
Characteristics of A.C. Synchronous Motors . 

Miscellaneous tests on the Effects of Self-Induction and 
Capacity in Alternating Current Series and Parallel 
Circuits ........ 

Effects of Position of Core, Air Gap, and of one Choker 
on another in open and closed Magnetic Circuits, etc. 

Measurement of Magnetic Permeability, Hysteresis, Self- 
Induction, and Iron Core losses .... 

Measurement of the Electrostatic Capacity of Concentric 
and other Cables ....... 

Measurement of Power in Single and Polyphase Alternating 
Current Circuits ....... 

Efficiency, Regulation, and other tests on Single and Poly¬ 
phase Alternating Current Static Transformers 

Efficiency and Regulation of Electrolytic and Rotary 
Rectifiers ........ 

Efficiency and regulation of Continuous and Polyphase 
Current Rotary Converters ..... 

Determination of the Periodic E.M.F. and Current Curves 
of an Alternator or A.C. Circuit by Joubert’s Contact 
Maker and the Duddell Oscillograph 

Efficiency and Performance of Ventilating Steam- and Gas- 
Engine Generating “ Sets ” . . . . . 

Jointing in Electric Light Wires and Cables . 

Appendix—Proof of Formulje used in the Tests 

Description of Appliances and Apparatus 

Tables of Constants, Variables, Copper and 
Resistance Wires, and Useful Numbers . 

Tables of Logarithms, Squares, Reciprocals and 
Doubled Square Roots of Numbers . 


Pikas 

232 

260 

299 

305 

310 

339 

345 

3G9 

379 

400 

427 

433 

446 

462 

473 

490 

504 

640 

657 

667 


Index 


ELECTRICAL ENGINEERING TESTING 

Curve Plotting. 

Introduction. —The practice of recording the results of any 
measurements or tests graphically, as well as in tabular form, in 
cases where this is possible, cannot be too strongly urged, and is 
a most important, as well as, in many cases, an indispensable 
operation. More especially is this the case with a large number 
of physical measurements, and particularly so with a majority of 
tests in Electrical Engineering. The practice of curve plotting, 
as these graphical representations may otherwise be termed, 
presents the following important features— 

(a) It enables the nature of the variation of one quantity with 
another to be seen at a glance much more clearly than is possible 
by aid of a table of results. 

(b) It enables errors in experimental observations, of which 
there are sure to be some, to be corrected comparatively easily, 
which in a majority of cases would be impossible from the table 
of results. 

(c) In the case of the calibration of instruments it enables 
the law of that under test to be readily observed. 

(d) It has the enormous advantage of enabling any inter¬ 
mediate value between those actually observed and tabulated, to 
be at once obtained accurately. This, it will readily be conceded, 
is the most important and valuable feature of all, and the ease, 
as well as the rapidity with which the operation of obtaining 
intermediate values can be accomplished, will be dependent on 
the scales chosen in originally plotting the curve in question. 

B 


2 


ELECTRICAL ENGINEERING TESTING 


It may therefore be profitable to indicate the mode of procedure 
in plotting curves, and with a view to exemplifying it, the results 
of a particular test are given in Table I., and the corresponding 
curve or graphical representation in Fig. 1. They relate to the 
determination of the Brake Horse Power (B.H.P.) of an electro¬ 
motor and the corresponding value of its efficiency at each load. 



B.H.P. 

Efficiency. 

1*5 

10 

1-75 

20 

3-5 

28 

4-0 

40 

6-0 

53 

8-0 

67 

11-5 

75 

16-5 

81 

21-0 

86 

30-0 

91 

35*0 

91 


In testing work generally at least six or eight different deter¬ 
minations throughout the range should be made, where possible, 
in order that the curve may be drawn more accurately. In most 
cases a curve constructed on three or four points only would be 
practically useless and could not be depended on. 

Directions for Plotting.— (1) Assuming that all the results 
have been worked out numerically and entered up in tabular 
form, the first thing to note is what two sets of quantities have 
to be plotted together, and secondly, the largest value of each 
set, for beyond this the scale need not extend. 

(2) The left-hand vertical and bottom horizontal sides OP and 
OQ respectively of the squared sheet of curve paper are termed 
the axes and are rectangular. They intersect in a point 0 called 




































































































































































ELECTRICAL ENGINEERING TESTING 


3 


the origin. Distances measured vertically are termed ordinates, 
and those horizontally, ahscissce. 

(3) Carefully note which set of readings have to be plotted on 
the ordinates and which on the abscissje, and then choose the 
scales of the axes OP and OQ such that they are as long as 
possible and include the maximum values to be plotted. Also, if 
possible, arrange such that one of the smallest divisions represents 
a simple whole number of one digit. For example, if 33 was the 
largest number to be plotted and the side of the squared paper 
contained 100 divisions, let 1 division represent 0 5 only, whence 
66 will give the 33; this is far more convenient a scale for 
future reference in obtaining intermediate values than 1 division 
representing 0*33 (i. e. 99 to give the 33 approx.). While it is a 
great advantage for the numerical length of the axes to be as 
large as possible, so as to enable the curve to be drawn larger 
and more accurately, the length should be decided by consider¬ 
ations of future reference to it for intermediate values as just 
mentioned. 

(4) The axes must be numbered every 10th division, and under 
no circumstances with the numbers obtained from experiment. 

(5) Write along each axis the nature of the quantity plotted on it. 

(6) Each point must be plotted by finding the point of inter¬ 
section of the axes representing the two corresponding quantities 
under consideration at the moment and a distinctive mark there 
made. 

(7) When all the points are plotted, a mean curve, as shown by 
the full line, Fig. 1, must be drawn through as many points as 
will allow of a uniform line being drawn. 

Some points are always sure to lie on either side of this mean 
line and denote experimental errors. The object of the curve 
is to correct for these. 

(8) In some tests, as for example in “ char¬ 
acteristic" determinations with direct current 
generators, it often happens that curves cross 
one another and lie close together. In such 
cases they must be drawn thin and a different 
notation for the respective sets of points 
used, such as that represented in Fig. 2. 

All confusion will thus be avoided. 


Kl gl ^ E 

H 0 H B B 

0 Q o - Q - - e - 

Fia. 2. 







4 


ELEGTRIGAL ENGINEERING TESTING 


Calibration and Standardization of 
Electrical Measuring Instruments. 

General Remarks. —This subject is perhaps one of the most 
important in connection with electrical testing, and we shall 
therefore devote some considerable attention to it. It will at 
once be obvious to any one, without any consideration, that a 
measuring instrument which is reading incorrectly, or one that 
has been calibrated so long ago that its present readings may not 
be true, is a useless instrument, or even worse than this, as one is 
unconsciously liable to take its reading as correct. The import¬ 
ance of correct reading and accurately calibrated measuring 
instruments cannot be over-estimated, for on their being so hangs 
the whole crux of further testing, the results of which would 
otherwise be quite worthless. This the author would emphasize 
most strongly, for it is unfortunately his experience, and no doubt 
that of many more like him, that the average experimentalist is 
only too ready to take the scale reading of any instrument as 
correct without in the least troubling himself as to whether it 
actually is so or not. This no doubt arises from the little extra 
trouble required to calibrate such instruments prior to starting 
some particular test. . 

Measuring instruments may change their constants and 
develop errors in their scale readings either from continued use, 
abuse, or in transit from one place to another, some of course 
being much more susceptible to alteration than others. Hence 
in all cases where it is desired to obtain accurate results and do 
good work, the instruments should be re-calibrated and re¬ 
standardized frequently, and a calibration curve drawn whenever 
possible with the date of the test inserted. At the very least six 
determinations should be made, wherever possible, but preferably 
ten or twelve, as it is not possible to draw a reliable calibration 
curve on less than six points. In all cases it is of the utmost 
importance to see that the connecting wires or cables do not 
magnetically alfect the instruments, for it must be carefully 
remembered that a wire carrying a current, no matter whether 
it is straight or otherwise, acts as a magnet. 


ELECTRICAL ENGINEERING TESTING 


5 


Such inductive effects will be minimized by running or twisting 
the “lead” and “return” together, when the two equal and 
opposite magnetic effects neutralize. An ordinary flexible twin- 
lead is non-magnetic externally, but it possesses a very small 
electrostatic capacity. 

Instruments are usually calibrated by comparing their readings 
with those of very accurately calibrated standard instruments. 
Simultaneous readings must be taken on both to avoid errors due 
to variation in between. Ammeters are always connected in 
sei'ies and voltmeters are always connected in 'parallel with their 
standards. 

In the calibration of voltmeters, the employment of keys in 
any of the branched or parallel circuits containing voltmeters to 
be calibrated is usually a source of inconvenience and should 
be avoided, for a key which places, say, a voltmeter of 1500 
Ohms resistance in parallel with a similar instrument already 
reading, will cause this reading to decrease owing to the alter¬ 
ation of the P.D. at the terminals due to inserting such a low 
resistance meter, and the consequent reduction in the terminal 
combined resistance. 

(i) Calibration of an Ammeter by comparison 
with a Standard D'Arsonval Ammeter, 

Introduction. —When a standard current measurer, such as a 
Kelvin standard balance or a potentiometer set, is not available for 
comparing the ammeter to be tested with, the following method 
of calibration may conveniently be employed. It consists in using 
a good reflecting D’Arsonval galvanometer in conjunction with 
a low resistance composed of platinoid or other suitable material 
having a small temperature co-efficient of resistance, which 
should preferably be known. The resistance of the D’Arsonval 
galvanometer may conveniently be something like 2000 to 4000 
times that of the low resistance to which it is shunted. The 
instrument, its scale, and the resistance should be permanently 
fixed and standardized carefully by means of a copper or silver 
voltameter. Then if the current which produces a full scale 
deflection, with a certain known resistance in series with the 
galvanometer, is accurately known, the current producing any 


c 


ELECTRICAL ENGINEERING TESTING 


other deflection with the same resistances will be very approxim¬ 
ately in direct proportion and therefore at once known. Some 
slight corrections might be necessary for great accuracy when 
subsequently using these particular constants, due to alteration 
of resistance through change of temperature*and to the deviation 
of the D’Arsonval readings from the direct proportional law, for 


which correction see Appendix, p. 490. 
Apparatus. —Secondary battery B 


r 



Fig. 3. 


capable of giving the 
maximum current required; 
reflecting D’Arsonval gal¬ 
vanometer (p.569); switch 
S ; key K ; carbon rheostat 
(p. 597); resistance box 
(r); ammeter A to be cali¬ 
brated; low resistance PP 
either of the form shown 
(p. 605), or simply a sheet 
of the metal. 

N.B.—It is assumed that the galvanometer and low resistance 
in combination has been carefully standardized previously and 
now constitutes the standard D’Arsonval ammeter. 

Observations. —(1) Connect up as in Fig. 3, and adjust the 
pointer of A to zero and the spot of light from G to the left-hand 
end of the scale used as a temporary or false zero in this 
test. 

(2) Insert the proper resistance in (?•) as given from the 
constants of standardization for the maximum current to be 
measured and corrected for the temperature of the room at the 
time of the test. 

(3) With R large, close S and adjust the current through the 
ammeter to be calibrated to about y\th of the maximum scale 
reading by means of R. Then note simultaneously its reading A 
and the deflection d on G when K is pressed. 

(4) Repeat 3 for about ten different readings on A rising by 
about equal increments to the maximum with no decreasings of 
current. 


(5) Repeat 3 and 4 for a similar descending set of the same 
readings on G, noting the corresponding ones on A, avoiding all 
increasings of current, and tabulate your results as follows— 

















ELECTRICAL ENGINEERING TESTING 


7 


Name . . . Date of test . , . 

Ammeter tested : No. . . . Type . . . Resistance (?•)= • • . Ohms. 
Tenii)erature of Room = . . . "C. Resistance of (ff)= . . . Ohms at . . . *0. 


Reading on Ammeter 
tested. 

Deflection 

on 

D’Arsonval. 

d. 

Corrected 
Readings of 
D’Arsonval. 
D. 

True Current 
i. e. (D) reduced 
(a) Amps. 

% Error of 
Ammeter tested. 

Ascending 

A. 

Descending 

A. 








(6) Plot curves having values of true current (a) as abscissae 
and A as ordinates. 

Inferences. —Enumerate any sources of error in ammeters 
generally. What can you infer from your experimental results 1 
Why should the current be so carefully increased only in 4 above 
and decreased only in 5 above ? 

(2) Calibration of an Ammeter by comparison 
with a Kelvin Composite Balance used 
as a Centi-ampere Meter. 

Introduction. —The following is a convenient and ready means 
of calibrating any ammeter reading up to 1 ampere, employing a 
Kelvin composite balance used in the manner mentioned, as a 
standard for comparison. A complete description of the con¬ 
struction and manipulation of the instrument will be found on 
p. 654, to which a reference should be made and the constants 
obtained therefrom. 

Apparatus. —Kelvin composite balance K.B. (p. 564) ; ammeter 
A to be tested; switch S ; 
adjustable resistance R (p. 

600, et seq.)-, source of cur¬ 
rent (7 at a P.D. of from 40 
to 60 volts. 

Observations.—(1) Con¬ 
nect up as in Fig. 4, ad¬ 
justing both instruments 
carefully to zero. Make 
quite certain that the con¬ 
nections are as indicated. 

(2) Turn the switch in front of the balance to “ volts so as 
to place the fixed and movable fine wire coils in series with each 




-o 





\ I 


C 




R 

'—orwji'- 


Fig. 4. 


































8 


ELECTRICAL ENGINEERING TESTING 


other and with the circuit. Now adjust the balance and its 
sensibility by employing the proper weights as given in the table 
of constants (p. 556), so that the maximum current to be measured 
on A would give a reading on E.R. as nearly right across the 
scale as possible. 

(3) With R as large as possible close S and obtain about 

of the maximum scale reading on A by varying R. Note this 
and simultaneously the corresponding position (d) of the slider 
of K.B. 

(4) Repeat 3 for about ten different values of current on A 
(by altering R) rising by about equal increments to the maximum. 

(5) Repeat obs. 4 for a similar set of descending values of 
current, and tabulate your results as follows— 


Namh . . . Date . . . 

Composite Balance used: No. . . . Constants = . . . Ammeter tested. . . 


Slider Reading 

True Amps. 

Reading on A. 

% Error of 

Mean % 

(d). 

K.d. 

Ascending. 

Descending. 

A. 

Error. 








(6) Plot a calibration curve for the ammeter tested having 
readings on A as ordinates and “ True Currents ’’ as abscissae. 

Inferences. —What sources of error are ammeters in general 
liable tol Can anything in particular be inferred from your 
experimental results ? 

(3) Calibration of an Ammeter by comparison 
with a Kelvin Composite Balance used 
as a Hekto-ampere Meter. 

Introduction. —The Kelvin composite balance can be used as 
a standard ammeter for the measurement of direct currents up 
to about 600 amperes, and hence any other ammeter can be 
readily calibrated by comparison with it. A description of the 
construction of the balance will be found on p. 554, together 
with the method of using it to measure heavy currents. In this 
connection it will be seen that the current to be measured passes 
through the thick fixed wire coils only, which act on the movable 















ELEGTRICAL ENGINEERING TESTING 


9 


coils of fine wire carrying a small auxiliary current from prefer¬ 
ably an independent source of E.M.F. The accuracy therefore 
of the calibration will depend on the accuracy with which the 
current through the moving coils is measured, and this is a 
disadvantage in the use of the composite balance for current 
measurement. 

Apparatus.—Kelvin composite balance K.B. ; ammeter A to be 
tested; low reading 
accurately calibrated 
ammeter {a) ; rheo¬ 
stats R (p. 597) and 
r (p. 600); switches 
and S 2 ; battery 
G capable of giving 
the current corre¬ 
sponding to the high¬ 
est scale reading on A. 

Note.—If a second battery, such as six small cells, is 
available the auxiliary current circuit may preferably be fed 
from it, for then this current will remain unalfected when the 
main current through A is altered. 

Observations.—(1) Connect up as in Fig. 5 if only one battery 
is used, and adjust the pointers of A and a to zero, and also that 
of K.B. in the manner mentioned on p. 556. Make quite certain 
that the connections are as indicated in the diagram. 

(2) Turn the switch on the balance (in front) to ‘^Watts’’ so as 
to put the fine wire movable coils only in connection with the 
small terminals, and therefore in series with the auxiliary circuit. 

(3) Adjust the balance and its sensibility by employing the 
proper weights as given in the table of constants (p. 557) so that 
the maximum current to be measured on A would produce as 
nearly as possible a full scale reading on K.B. 

(4) With r at its maximum close S 2 and adjust the current 
through the fine wire movable coil to its proper value, as given 
with the constantf by varying r. Always make quite sure that it 
has this value before taking each reading on K.B. 

(5) With R large, close and obtain about y^h of the 
maximum scale reading on A by varying R. Note simultaneously 
the reading on A and the position {d) of the slider of K.B. 




























10 


ELECTRICAL ENGINEERING TESTING 


(6) Repeat 5 for some ten different values of current on A 
(by altering R) rising by about equal increments to the maximum. 

(7) Repeat obs. 6 for a similar set of decreasing currents and 
tabulate as follows— 


Name . . . Date . . . 

Composite Balance : No. . . . Constants used . . . Current in Moving Coils = . . . Amps. 
Ammeter tested . . . Tyjie . . . No. . . . Range . . . 


Slider Beading 
(d). 

True Amps. 
K.d. 

Reading on A. 

% Error of 

A. 

Mean % 
Error. 

Ascending. 

Descending. 








(8) Plot “ calibration ” curves for the ammeter tested having 
readings on A as ordinates and true currents as abscisste. 

Inferences. —What sources of error are ammeters in general 
liable to? Can you infer anything in particular from your 
experimental results ? 


(4) Calibration of an Ammeter by comparison 
with a Kelvin Centi-ampere Balance. 

Introduction. —Any ammeter reading up to 1 ampere can be 
readily calibrated by comparison with a Kelvin standard centi- 
ampere balance. A full description of this instrument, together 
with the table of constants, will be found on p. 546, and the 
method of using it is precisely the same as that of the composite 
balance used as a centi-ampere meter, except that there is no 
switch at all on the balance in question. The operator should 
refer to the Appendix (p. 546) for details in connection with the 
balance. 

Apparatus, —This, with the exception of the balance, is precisely 
the same as is required for the corresponding calibration by the 
composite balance. 

Observations. —These, together with the diagram of connections 
and the inferences, are precisely the same as for the test on p. 7, 
and will not therefore be repeated here. The operator must 
refer to the similar test using the composite balance. 














ELECTRICAL ENGINEERING TESTING 


11 


(5) Calibration of a Direct Current Ammeter. 
(Crompton Potentiometer Method.) 

Introduction. —This method is a very convenient and accurate 
one for calibrating ammeters, and in it the measurements are 
referred to and obtained in terms of a standard Clark cell and 
standard resistance. The principle of the method is a direct 
application of Ohm’s Law, and consists in measuring the fall of 
potential down an accurately known standard low resistance 
connected up in series with the circuit through which the current 
to be measured is passing. This fall of potential is measured in 
terms of the E.M.F. of the Clark’s cell through the medium of 
the potentiometer, employing the principle of the Clark-Poggen- 
dorff method for comparing two or more E.M.F.’s. A detailed 
description of this will be found in a separate work by the author 
on P7'actical Electrical Testing for 1st and 2nd year students 
and others. The Crompton potentiometer is a specially arranged 
form of comparing instrument by means of which the calibration 
can be easily and quickly carried out. Before proceeding further 
the operator should refer to a detailed description of this piece 
of apparatus which will be found in the Appendix (p. 510) together 
with the method of using it. The present method possesses the 
all-important advantages that the measurements are all in terms 
of the Official Board of Trade standard—the Clark cell—that 
their accuracy is great, and without any very special means 
this can be obtained to at least 1 in 1000, and that the range 
is almost illimitable from 0 continuously up to maximums 
commonly met with in practice. The accuracy of the results 
in the present method is more particularly dependent on that 
with which the standard low resistance is known. The value 
of this must be such that the fall of potential down it due 
to the maximum current to be measured is not greater than LS 
volts, while at the same time the carrying capacity must be such 
as to allow it to pass this current without sensible heating, which 
would thereby alter its resistance. 

Apparatus.— Crompton potentiometer P (p. 510); secondary 
battery B capable of easily giving the maximum current required 
for a full scale reading on the ammeter A to be calibrated; 


12 


ELEGTBIGAL ENGINEERING TESTING 


switch S; one secondary cell (5) for the “working cell” of the 
potentiometer; accurately known standard low resi-stance R 
(p. 605) ; sensitive D’Arsonval or moving coil galvanometer (y)* 
(p. 569); standard Clark cell E ; carbon rheostat (rA) (p. 597). 



Observations.—(1) First place the levers of G and E (Fig. 208) 
on studs 14, and that of H on studs 1 or 2 for precaution. 
Now connect up as in Fig. 6, in which only the row of ter¬ 
minals on the potentiometer P is shown symbolically for brevity. 

(2) Adjust the galvanomter g approximately, and A carefully, 
to zero, levelling them if necessary. See that a suitable low 
resistance standard R is employed, and one that will give a fall of 
potential at its ends not exceeding 1*5 volts {= A x R) for the 
maximum current to be used (vide p. 13). 

(3) “ Set the potentiometer ” by the standard cell in the way 
described on p. 514, the contact lever II above referred to being 
on studs IV, thus inserting E (Fig. 6) in the circuit of g, which 
must be done so that its E.M.F. opposes that of h. Now close 
S and adjust rh so as to obtain about yV^h of the full scale 
reading on A. 

(4) With the positions of the resistances G and (Figs. 207 
and 208) as found in 3, uncdtered, turn the lever of H to studs III 
so as to throw into circuit with g the fall of potential down R 
which as seen is across terminals III. Now adjust the lever of 
the resistances E (Fig. 207) and the sliding key G so that no ■ 
deflection occurs on g on pressing this latter. 

N.B.—If it is impossible to get a balance owing to the deflection 
of g being always to one side, the P.D. across III is assisting, 
instead of opposing (as it should be) the fall due to h in the 










ELECTRICAL ENGINEERING TESTING 


13 


stretched wire; the wires from R to III must then be inter¬ 
changed. 

If the lever at E is on stud 1 (literally 1000) and the slider G 
at 315 on the scale, the P.D. across R = *1315 volts and the true 

•1 31 ^ 

current A-^ through ^ if ^ = 0*1 Ohm is — =1*315 amps. 

Note simultaneously the readings on P and A when balance is 
obtained. Turn H to lY again and see whether the balance in 
obs. 3 still holds. If it does not, re-set P. 

(5) Take about ten different scale deflections on A rising by 
about equal increments to the maximum by varying (r/i) and 
repeat 4, noting the new values of P and A, 

(6) Kepeat 4 and 5 for a similar descending set of readings on 
A and tabulate your results as follows— 


Namb . . . Datb . . , 

Clark Cell: No. . , . Temperature =. . . ® C. : E.M.F. assumed = . . . volts. 
Potentiometer setting: on . . . C at . . . Standard Low Resistance R = . . . Ohms. 


Ammeter 
Reading A. 

Potentiometer Reading. 

True P.D. 
across (A) 

r. 

True Current 

y 

Ai = Lamps. 

Error of 
Ammeter 
(Ai-A). 

X Error. 

Stud of E. 

Position of 
Slider (C). 









It should be carefully noted whether a “ Clark ” cell or 
“ Carhart-Clark cell is being used before setting the potentio¬ 
meter in 3, and that the assumed E.M.F. for this purpose at the 
particular temperature is correct, the temperature coefficient of 
E.M.F. being very different in these two cells {vide p. 643). 

Standard Weston (cadmium) cells have an international 
value of E.M.F. of 1’0183 volts at 20° C., with a temperature 
coefficient of - 0'0000398 volt per rise of 1° C., between 0° and 
40° C. In 1908, for a range of 0° C. - 40° C., Wolff obtained 
the relation giving the E.M.F. at °^ C., namely—• 

^20 - 0-0000406(^ - 20) - (9*5 x 10-^)(« - 20)2 
= 1*0183{1 — 0'0000398(< - 20)} volts approx, (see table 643) 

(7) Plot calibration curves for the ammeter tested having values 
of A as ordinates and true amps A^ as abscissae. 

Inferences. —What can you infer from your experimental 
results, and can you suggest any sources of error which might 
vitiate the results? 


















14 


ELECTRICAL ENGINEERING TESTING 


(6) Determination of the “Constant” of a 
Galvanometer or Ammeter by Means of 
a Copper Voltameter. 


Introduction. —From the results of a large number of tests it 
has been found that, using the necessary precautions, the constant 
of an electric current instrument can be obtained with certainty 
to within of absolute accuracy by the electrolysis of copper. 
The voltameter (F) should consist of three or more pure copper 
plates dipping into a saturated solution of copper sulphate con¬ 
tained in a suitable glass or earthenware vessel, there being one 
more “anode” than “cathode,” and the two sets arranged altern¬ 
ately with an anode at each end. The plates should be as square 
as possible, and placed from ^ to f" apart; if too close, polariz¬ 
ation will take place when strong currents are used, and the 
current density (reckoned in amps, per sq. cm.) is too great. 
There should not be less than 30 sq. cms. per amp.; if there is, 
the plate surface will be too small and the deposit on the cathode 
irregular, some of it falling to the bottom of the vessel. The re¬ 
sistance of V will also become high and variable, due to the form¬ 
ation of copper oxide, and will give trouble in keeping the current 
constant. The solution should be a saturated one (sp. gr. 1"211) 
of pure copper sulphate crystals and distilled water, with 1% 
by vol. of strong sulphuric acid added, which is necessary to 
insure success. The vol. of solution should be about 1100 cc. 
per amp. The anodes may be made of about No. 18 S.W.G., 
and the cathodes or gain plates of No. 30 S.W.G. pure copper, 
all edges and corners being smooth and rounded. The electro¬ 
chemical-equivalent (Z) of any substance, in this case copper 
= No. grms, deposited by 1 coulomb. 



Fia. 7. 


Table II. 


Cathode area 
in sq. cm. 
per amp. 

Values of (Z) for Copper. 

2° C. 

12° C. 

23° C. 

30 

0-0003289 

0-0003290 

0-0003289 

50 

88 

87 

86 

100 

88 

84 

83 

150 

87 

81 

80 

200 

85 

79 

77 

250 

83 

78 

75 

300 

82 

78 

72 

















ELECTRICAL ENGINEERING TESTING 


15 


Apparatus.— Yoltameter (F); rheostat R (p, 597); switch S; 
ammeter A to be standardized; secondary battery; drying cup¬ 
board D, not shown; acid bath. 

Observations.—(1) Connect up as indicated in Fig. 7, and 
adjust A to zero. Light the gas-jet under the steam boiler of i), 
after seeing that the latter contains enough water. 

(2) Determine the necessary area of cathode, and hence the 
number of gain plates required for the current to be used, 
reckoning both sides in contact with liquid as the effective area 
of cathode. 

(3) Carefully clean the cathodes all over with fine emery cloth 
until quite bright, then dust with a (Ly deem dotli^ and do not 
toudh the part to he immersed with the fingers. Clean the anodes 
if they look dirty. 

(4) Carefully weigh the gain plates on a chemical balance to 
1 m.g., and note their weights (IFj) grams. 

(5) Insert the same area of trial plate to act as cathode, so as 
to adjust the current to the value required, then remove them, 
making sure that the -f of battery is joined to anode. 

(6) Insert the weighed gain plates, and at a convenient and 
noted instant of time switch on, quickly adjusting the current to 
its proper value. 

(7) Keep it flowing for at least thirty minutes, and maintain it 
constant all the time by altering A", if necessary. (Kote.—1’177 
grams of copper (cupric) are deposited per amp.-hour approx.) 

(8) Note the exact instant of switching off. Yery carefully 
remove the gain plates so as not to scratch them, rinse in acidu¬ 
lated water to prevent the nascent copper oxidizing, then in clean 
water, and place in D to dry. 

(9) When dry and cool re-weigh the gain plates and note the 
weights TFg grams. 

(10) Repeat 2-9 for one or two other current strengths and 
tabulate as follows— 


Name . . . Date . . . 

Cathode Area = . . . sq. cnis. per amp. Temperature of Bath = . . . ® C. Z = . . , 


Weight of j)lates in 
grains. 

Deposit W 
=(2Jr2-2JFi) 

Time in Secs. 
(0 

Reading of 

A. 

True Amps. 
W 

% Error of 

A. 

Before XIFi 

After SIFo 

Zt. 
























IG 


ELECTRICAL ENGINEERING TESTING 


(7) Calibration of Direct Current Voltmeters. 
(Poggendorff's Method.) 

Introduction. —The following method is a convenient one for 
enabling low-reading voltmeters up to about 3 or 4 volts to be 
easily and rapidly calibrated by comparison with one or two 
Clark’s standard cells. Some convenient form of metre bridge, 
either circular or ordinary, is required. The principle of the 
method will be seen to be practically the same as the Clark- 
Poggendorff ” method of comparing two E.M.F.’s. 

Apparatus. —Metre bridge of some convenient type, either the 
ordinary straight or circular form ; secondary battery B giving 
an E.M.F. a little in excess of the maximum voltage to be recorded 
on the voltmeter V to be calibrated; key h ; carbon rheostat 
R (p. 597); sensitive galvanometer G (p. 572) ; high resistance ( 7 ’) 
of about 10,000 ohms; standard cell {S) of known E.M.F. It 
will be observed that the metre bridge, of whatever form is used, 
is represented symbolically by PQ in Fig. 8, and if the general 
scheme of the connections is understood, there will be no difficulty 
with them when using any form of bridge. 



Observations. —(1) Connect up as in Fig. 8, adjusting V care¬ 
fully to zero and G to about zero. See that like poles of B and /S 

























IJLECTRIGAL ENGINEERING TESTING 1? 

are connected to the same end P, when the respective E.M.F.’s 
will oppose one another. 

(2) Adjust P to a high value and likewise (?’), which might 
preferably be about 10,000 ohms, and is merely for the purpose 
of preventing the standard cell S from sending but a very minute 
current when its circuit is closed at the tapping key or rolling 
contact (7, which at first might be far from the correct position 
of balance. When G is moved to such a position that the 
defiection on G is small, (r) may temporarily be cut out of circuit 
so as to make the sensitiveness of the test a maximum. 

(3) Now close K and alter R so as to obtain about of 
maximum scale reading on V. 

(4) Find a position for 0 such that on making contact with 
the bridge wire by means of it, no defiection occurs on Gj r being 
manipulated as in obs. 2. Note the reading on the voltmeter V 
calibrated and the position of G where PG = 9\ and QG = 

(5) Re-insert r and repeat obs. 3 and 4 for about ten readings 
on V rising by about equal increments to the maximum. 

(6) Calculate the true volts Fj- from the relation— 

Vx = X E.M.F. of standard cell, 

ri 

and tabulate your results as follows— 


Namr ... Date ... 

E.M.F. of Standard CeU = . . . Volts at . . . <» C. r =. . . Ohms \ 



ri-fr2 

Reading on 

V. 

True Volts 

% Error of Voltmeter 
tested. 







Note.—The E.M.F. of a Clark’s standard cell = 1’4340 legal 
volts at 15° C., and its E.M.F. at other temperatures may be found 
from the relation— 

»E.M.F. = 1'4340{1 - 0-0007 (( - 15°)} legal volts, 
where 0-0007 is the temi^eratuTQ coefficient of the cell and 
t = its temperature in degrees Centigrade. For a Carhart-Clark 
cell the coefficient is 0-00038. For table of E.M.F.’s see p. 643. 

(7) Plot a calibration curve of the voltmeter under test having 
values of V as ordinates and Vx as abscissae. 

Inferences.—Does the accuracy of the above test depend on 


0 













18 


ELECTRICAL ENGINEERING TESTING 


anything in particular 1 Show how the relation given in 6 can 
be obtained and state any assumptions made in deducing it. 


(8) Calibration of a Voltmeter by Comparison 
with a Standard D’Arsonval Voltmeter. 


Introduction. —The method may conveniently be employed for 
calibrating a voltmeter when neither a Kelvin standard balance 
nor a “ potentiometer set ” is available for use as a standard with 
which to compare the instrument under test. In the present case 
a fairly sensitive and good form of D’Arsonval galvanometer 
combined with a high resistance placed in series constitutes the 
standard voltmeter which together with its scale is permanently 
fixed up. The arrangement is very carefully standardized and 
its constants found with the aid of a Clark’s standard cell and 
Clark’s potentiometer method (p. 16), and thus a reliable standard 
voltmeter is obtained. 

If the voltage which produces a full scale deflection with a 
certain resistance in series with the instrument at a given tem¬ 
perature is known, then that causing any other deflection under 
the same conditions will be in direct proportion and therefore at 
once known. For considerable accuracy some small corrections 
would be necessary in using these constants at some other time 
owing to the difference in temperature altering the resistances of 
the galvanometer coil and those in series with it, and to the 
D’Arsonval not exactly fulfilling the direct proportional Law, 
for which correction see p. 490. 


The D’Arsonval and its 
resistance can be arranged 
in one of two ways— {a) 
as represented in Fig. 9, 
assuming that a sufficiently 
high adjustable known re¬ 
sistance for placing in 
series with it is available. 
If then the “ figure of 
merit ” of G, i. e. the am¬ 
peres (a) per scale division, is accurately known, the extra high 
resistance (r) necessary to be placed in series with it so that the 



Fig. 9. 
















ELECTRICAL ENGINEERING TESTING 


19 


required maximum voltage applied to AI) (Fig. 9) may produce a 
full scale deflection on Gj can be found by Ohm’s Law as follows— 
If {d) = the maximum scale reading to be obtained by a 
maximum voltage V, then if (g) = the resistance of the galvano¬ 
meter (?, we have (?’ -t- = iT _ 

c ad 

{h) If G is very sensitive and sufficient resistance is not avail¬ 
able the arrangement in 
Fig. 10 may be used, r 
now taking the form of 
twm resistance boxes 



and 7*2. 

In this case will be 
small compared with the 
resistance of G and with 
(7*1 4 - 7 * 2 ), and this sum 
large compared with the 
resistance of V, the voltmeter to be calibrated. 

If Fand V are the voltages across HD and 7*j respectively and 
(g) the resistance of G, whose “ figure of merit ” is (a), 

then V : V = 7*2 4- -^aJL : 

ri + 9 r^+9 

and if {d) has the same meaning as before v = acZy, 

whence V — = ad 9 (?•„ 4- • ^ 1 ^ ^ 

^1+9 \ '^' 1 + 9 / 

assuming t’j to be negligibly small compared with 9 and 7*2 we 

9* oJXO 

have -i- = approximately, which is obvious from a considera- 

7*2 F 

tion of Ohm’s Law. 

Referring to Fig. 8 it will at once be seen that discarding the 
cell aS’, the standard D’Arsonval G with its extra resistance 7 * 
may be employed to actually measure the P.D. between P and G^ 
whence knowing the ratio of PQ to PC the true volts corre¬ 
sponding to any reading on F (Fig. 8) can at once be obtained. 
Thus the arrangement just referred to practically brings us to 
that shown in Fig. 10. 

Apparatus. —Secondary battery R of a sufficient number of cells 























20 


ELECTRICAL ENGINEERING TESTING 


to give the highest reading on the voltmeter (F) to be calibrated ; a 
fairly high resistance D’Arsonval galvanometer G (p. 569); spring 
tapping key k ; one or two high resistance boxes and rg; 
variable unknown high resistance rheostat R ; this latter, how¬ 
ever, will be of no use if the resistance of the rest of circuit 
is very high, and in this case the voltage due to B will have to 
be varied by altering the number of cells in the battery. ^ 

Observations.—(1) Connect up as shown in Fig. 10, which latter 
arrangement will be found to be the one most common in practice. 
Adjust the spot of light of G to the left-hand end of the scale 
as a temporary or false zero so as to obtain a full scale deflection 
up to the other end for the maximum voltage to be measured. 

(2) Insert the proper resistances in and rg as given from the 
constants of standardization for the maximum voltage to be 
measured and corrected for the temperature of the room at the 
time of the test. 

(3) Close k and adjust 7?, or alter the number of cells in B so 
as to obtain about yV^h of the maximum scale reading on Y and 
note simultaneously the readings on Y and G. 

(4) Repeat 3 for about ten different readings on Y rising by 
about equal increments to the maximum. 

(5) Repeat 3 and 4 for a similar descending set of the same 
readings on G, noting the corresponding ones on F, and tabulate 
your results as follows— 

Name . . . Date . . . 

Voltmeter: No. . . . Type . . . Resistance . . . Ohms. 

ft ^ 2 = . . . t, 

Temperatiu-e of roomss . . . °C. Resistance of . . . Ohms at . . . °C. 


Reading in Volts on Voltmeter tested. 

Deflection on 
D’Ajsonval 

D. 

True Voltage, i. e. 

J) reduced 
(v) Volts. 

X Error of 
Voltmeter 
tested. 

Ascending 

F. 

Descending 

V. 







^ Instead of the variable high Resistance rheostat (E) shown in tests 8-11 
which may not be available, the following arrangement for varying the 
voltage across the parallel combination of voltmeter tested and standard, 
will be found convenient, namely—connect two variable lamp resistances 
■El E^ in series across the supply and shunt one of them (/2j) with the volt¬ 
meter and standard in parallel, the lamps of each rheostat being operated 
in parallel and being for a voltage = to that of the supply. Then by keeping 
1 lamp in Ei and varying E 2 , the volts across Ei can be varied from J that of 
the supply to its full value, while by keeping 1 lamp in E^ and varying Ei 
the volts across Ei can be varied from ^ that of the supply to 0; thus 
covering the full range of supply volts on Ei. 













ELECT RIGAL ENGINEERING TESTING 


21 


(6) Plot “ calibration curves ” having values of true voltage 
{v) as ordinates and V as abscissae. 

Inferences. —Enumerate any sources of error in voltmeters 
generally. State clearly what you can infer from the results of 
your tests. 

(9) Calibration of a Voltmeter by Comparison 
with a Kelvin Composite Balance used as 
a Voltmeter. 

Intreduction. —The composite balance when used in con¬ 
junction with separate anti-inductive resistances may be con¬ 
veniently employed as a standard direct or alternating current 
voltmeter capable of measuring pressures up to about 600 volts. 
The following method assumes the use of such an instrument, and 
the reader should refer to p. 554, et seq., for the construction and 
mode of using this form of balance and for the table of constants 
and sensibilities. 

Apparatus. —Adjustable, fairly high resistance R (p. 603) j 
switch S; voltmeter F to 
be calibrated; Kelvin Com¬ 
posite Balance K.B.y with 
its non-inductive resistance 
9 * (p. 553); battery of second¬ 
ary cells capable of giving 
the maximum voltage to be 
used, which we therefore 
assume to be direct. 

Observations. — (1) Connect up as in Fig. 11, adjusting both 
instruments carefully to zero, and make quite certain that the 
connections are as indicated. 

(2) Turn the switch in front of the balance to “ Yolts ” so as 
to place the fixed and movable fine wire coils in series with one 
another across the terminals. Observe that the anti-inductive 
resistance r is numbered the same as, and therefore belongs to 
the balance in use, and make quite sure of having the correct 
resistances in it in use (p. 556). 

(3) Adjust the balance and its sensibility by employing the 
proper weights as given in the table of constants (p. 556), so that 





















22 


ELECTRICAL ENGINEERING TESTING 


the maximum voltage to be measured on V would produce as 
nearly as possible a full scale reading on K.B. 

(4) With R as large as it can be, close S, and obtain about 
Y^^th of the maximum scale reading on Fby altering i?. Note 
simultaneously the reading on V and position (d) of the slider 
of K.B. 

(5) Repeat 4 for some ten different values of voltage on V 
(by altering R or the number of cells in the battery) rising by 
about equal increments to the maximum. 

(6) Repeat obs. 5 for a similar set of decreasing voltages, and 
tabulate your results as follows— 


Name . . . Date . . . 

Composite Balance: No. . . . Constants . . . Voltmeter tested. . . . Temp. = . . . *0. 


Slider reading 
d. 

Volts 

K.d. 

Corrected 
Volts Ft. 

Reading on V. 

% Error of 

V. 

Ascending. 

Descending. 








N.B.—The values Vt&tq the readings on the standard corrected for temperature. 


(7) Plot a calibration curve for the voltmeter tested having 
readings on V as ordinates and true volts Vj- as abscisste. 

Inferences. —Wliat sources of error are voltmeters in general 
liable to ? Can anything in particular be inferred from the above 
results ] 

(lo) Calibration of a Voltmeter by Comparison 
with a Kelvin Centi-ampere balance used 
as a Voltmeter. 

Introduction.— The Kelvin Standard Centi-ampere balance 
when used in conjunction with extra anti-inductive resistances 
constitutes a most convenient standard voltmeter with which to 
compare any other voltmeter to be calibrated, up to about 800 
volts. 

For larger voltages, up to 2500 volts, special non-inductive 
high resistances are provided for inserting in series with the 
coils of the instrument. The combination then constitutes a 
standard voltmeter by means of which any other voltmeter, either 
for direct or alternating currents, reading up to 2500 volts, can 
be calibrated, by comparison, in the ordinary way. 

A complete description of this balance, together with the 














ELEGTRIGAL ENGINEERING TESTING 


23 


method of using it, will be found on p. 546 et seq., where the 
table of constants is given. 

In the present test the apparatus, observations and inferences 
are precisely similar to that of the corresponding calibration by 
means of the composite balance used as a voltmeter, the centi- 
ampere balance being substituted for this latter. They will 

consequently not be repeated here, but may be seen on p. 21. 

» 

(ii) Calibration of a Direct Current Voltmeter. 
(Crompton Potentiometer Method.) 

Introduction. —The method is a very convenient and accurate 
one for the purpose, and consists in calibrating the voltmeter to 
be tested in terms of the E.M.F. of a Clark’s standard cell, a 
known fraction only of the voltage applied to the instrument 
being actually compared with the standard E.M.F. The principle 
of the method is identical with that of the Clark-Poggendorff ” 
method for comparing two or more E.M.F.’s, a full description of 
which will be found in a separate work, by the author, on 
Practical Electrical Testing^ for 1st and 2nd year students and 
others. The Crompton Potentiometer is a specially arranged 
form of comparing instrument by means of which the calibration 
can be easily and quickly carried out. A detailed description of 
it will be found on p. 510, to which the reader should in the first 
instance refer. There are three extremely important features in 
connection with the present method, using the potentiometer ; 
firstly, the enormous range of applicability, for the instrument 
can be used equally well in the measurement of current and 
resistance as well as voltages from 0 to almost any practical 
amount; secondly, the measurements are in terms of the official 
Board of Trade Standard—the Clark cell—though any other 
standard can be used ; thirdly, the accuracy is great and under 
ordinary conditions the measurements are accurate to at least 
tV%> care, using a very accurately adjusted instrument, 

accuracy to something like can be obtained. In this form 
of potentiometer the highest voltage which can be compared 
directly is 1*5 volts, and hence the fractions employed of all 
higher pressures to be measured must fall within this limit. 

Apparatus. —Crompton potentiometer P (Fig. 208); secondary 
battery B capable of giving the maximum voltage required for a 


24 


ELEGTBIGAL ENGINEERING TESTING 


full scale reading on the voltmeter V to be calibrated; key or 
switch S; one secondary cell {b) for the “working cell” of the 
potentiometer; “ volt box,” i.e. divided resistance acd for obtaining 
a fraction (less than 1-5 volts) of the total P.D. to be measured 
(p. 521); sensitive D’Arsonval or moving coil galvanometer (y) 
(p. .569); standard Clark cell E; fairly high resistance rheostat R 
(p. 603). See footnote, p. 20. 



Observations. —(1) After placing the levers of G and E on 
studs 14 and that of II on studs 1 or 2 for precaution, connect 
up as in Fig. 12, in which only the row of terminals on the 
potentiometer P is shown, symbolically, for the sake of brevity. 

(2) Adjust the spot of light of the galvanometer to somewhere 
about zero on the scale, and the resistance ad and cd in the “ volt 
box ” so that cd — of ad, supposing, of course, that not more 
than 150 volts is across ad. Carefully level and adjust V to 
zero if it requires it. 

(3) “ Set the potentiometer ” by the standard cell in the way 
described on p. 514, the contact lever//, Fig. 208, p. 512, being on 
studs IV, thus inserting E in the circuit of g in such a way as 
to oppose that of h, close S, and adjust R so as to obtain the full 
scale deflection on F. 

Note.—This last operation will only be possible when R is 
comparable with the parallel resistance of ad and F. If both 
these are very high then altering R will have very little effect on 
the reading of F unless R is high also in comparison. 

(4) With the positions of the resistances G^ and G (Fig. 208, 
p. 512), as found in 3, unaltered, turn the lever of H to studs 















ELECTRICAL ENGINEERING TESTING 


25 


III so as to throw into circuit with g the y^th part of the 
voltmeter P.D., which was across terminals///. Now adjust the 
lever of the resistances E, Fig. 208, and the sliding key C, Fig. 
208, so that no deflection occurs on pressing this latter. 

N.B.—If no balance can be obtained owing to there being no 
reversal of deflection on g, the fractional P.D. across III is 
assisting instead of opposing (as it should be) the fall of potential 
due to (6) in the stretched wire. The wires from cd to III must 
then be interchanged. If the lever at E is on stud 12 
(literally 12,000) and the slider C at 625 on the scale, the voltage 
across ad, i. e. at the terminals of F=126’25 volts. Note these 
positions on PP, and simultaneously the reading V on the instru¬ 
ment to be calibrated. 

(5) Reduce V by about either by cutting out cells in B or 
by altering R or both, and repeat 4, noting the new values of 
V and PP. Turn H to IV for a moment and see whether the 
balance in obs. 3 still holds, if not re-set as in obs. 3 above. 

(6) Repeat 4 and 5 for some ten or twelve different 
readings on V decreasing by about equal amounts to the lowest. 

(7) Repeat 4 to 6 for a similar ascending set of observations, 
and tabulate your results as follows—• 

Name . . . Date . . , 

Clark Cell: No. . . . Temperature = . . . ° C; E.M.F. Assumed = . . ; Volts. 

* 

Volt Box: Fraction of total used — ±= ... Potentiometer setting E on .. . Cat. . . 

ad 


Voltmeter 

Reading 

V. 

Potentiometer Reading, 

True Volts Across. 

Error of 
Voltmeter 
Fi- F. 

% Error. 

Stud of E. 

Position of 
Slider C. 

Fraction cd 

V. 

Voltmeter 

V. 

cd 









As it may sometimes be the case that a Carhart-Clark and not 
a Clark’s standard cell has to be used, care should be taken that 
the E.M.F. assumed at the particular temperature is correct, the 
temperature coefficient of E.M.F. being very different in the two 
cases {vide pp. 17 and 643). 

(8) Plot a calibration curve for the voltmeter tested having 
values of V as ordinates and true volts as abscissie. 

Inferences. —What can you infer from the results of your test X 
Are there any sources of error which might vitiate the results X 



















26 


ELEGTBIGAL ENGINEEBING TESTING 


(12) Calibration of a High Tension Alternating 

Current Voltmeter. 

Introduction. —In the ordinary high tension systems of dis¬ 
tribution of electrical energy by alternating currents, the average 
working pressures are about 2000 or 2500 volts. The “electro¬ 
magnetic’’ and “hot wire” types of voltmeters are unsuitable 
for measuring such high pressures, which can best be dealt with 
by means of a third class of instrument known as the electro¬ 
static voltmeter, a description of two forms of which will be 
found in the Appendix (p. 562). 

These instruments are almost universally employed for measur¬ 
ing alternating current pressure, and they have the all-important 
advantage of being unaffected by variation of frequency. Owing 
usually to the difficulty experienced in obtaining direct current 
pressures of the above magnitude for testing purposes, alternating 
currents have nearly always to be employed for calibrating high 
tension voltmeters. Thus it will be seen that none of the pre¬ 
ceding methods are available in the present case, but the 
calibration can be effected by what may be termed the “ fractional 
potential difference ” method, using an accurately calibrated low 
tension electrostatic voltmeter for comparison in the manner to 
be described later. 

This low tension voltmeter, which may conveniently be one 
of Lord Kelvin’s multi-cellular electrostatic instruments redding 
to, say, 150 volts, should be very carefully calibrated by one of 
the preceding methods—preferably the potentiometer, one using 
a Clark’s cell as a standard of E.M.F. and direct current pres¬ 
sures of course. For accurate work, however, the following 
remarks should be observed. In these voltmeters the movable 
needle system is usually aluminium and the fixed system brass, 
whence owing to aluminium being electro-positive to brass, the 
instrument will read from 0-2 volt to 0-3 volt too low when the 
+ ’^® pole of the direct current source is connected to needle, and 
the same amount too high if the —is joined to the needle system 
instead. In calibrating this low reading voltmeter by the 
potentiometer, it should be connected up through a reversing key 
to the rest of the apparatus and the mean of the readings, before 


ELECTRICAL ENGINEERING TESTING 


27 


and after reversing the polarity on its terminals, taken as the 
correct one for alternating currents, since when used with such 
the above-named error does not occur. Again with direct currents 
an electrostatic voltmeter passes no current, but owing to it 
possessing a perfectly definite though very small capacity, it will 
behave like a condenser with alternating currents, i. e. a pulsat¬ 
ing or “ charge and discharge ’ ’ current will be set up in its 
circuit. Thus it will be seen that if, as in the present method, 
such an instrument is shunted across part of a circuit carrying 
an alternating current, the current in the voltmeter branch may 
be quite comparable with that in the main circuit, in other words 
the P.D. between the points to which it is shunted would be 
lowered somewhat by the voltmeter, and would not bear to 
the whole P.D. the ratio of the resistance of the two portions of 
the circuit. To avoid such an error the resistance of the main 
circuit should be such that the maximum pressure to be used 
sends a sensible current such as from to an ampere through 
it, which will consequently be very large compared with the 
current in the voltmeter branch. Thus the presence of this latter 
will not affect the value of the P.D. between the two points to 
which it is applied, and consequently the ratio of the whole P.D. 
to the fraction thus tapped will equal the ratio of the whole 
resistance to the fraction across which the electrostatic voltmeter 
is placed. 

Apparatus. —Alternator A, capable of supplying a low pressure, 
and of being driven at 
any required speed by 
a direct current, electro¬ 
motor (preferably), or 
other prime mover, its 
exciting circuit E con¬ 
sisting of the field coils 
of A (shown), together 
with switch, rheostat, 
ammeter and source of 
excitation (not shown), but which can be varied so as to vary the 
E.M.P. of A; step-up transformer T capable of increasing the 
pressure from that of A to the maximum required for a full 
scale reading on the high tension voltmeter F to be calibrated; 
















28 


ELECTRICAL ENGINEERING TESTING 


low tension electrostatic voltmeter (r) (p. 562), reading to say 
150 volts, and which has been previously carefully calibrated by 
reference to a standard Clark cell on the potentiometer ^ switch 
8 . A divided non-inductive high resistance (acb) capable of 
standing the highest pressure to be used on V, and of carrying 
an appreciable current, say of the order of to ^ an ampere 
continuously without excessive heating. 

Caution. —Under no circumstances whatever is any part of the 
high tension circuit to be touched while “alive,” and the india- 
rubber gloves are to be worn throughout the test by the operator 
reading the electrostatic voltmeters. 

Observations. —(1) Connect up as in Fig. 13, and carefully 
level and adjust the pointers of V and v to zero. For the high 
tension side use well-insulated wires for the connections, and 
keep them in mid-air as much as possible. 

(2) If the voltmeter V to be calibrated reads up to, say, 2500 
volts, and r to only 150 volts, place this latter across a convenient 
fraction (a c), say ^V^h of the whole non-inductive resistance (ah), 
which in this case may conveniently be something like 5000 
Ohms. 

(3) Start the alternator A, close s, and then adjust the speed 
and excitation so as to obtain the lowest scale reading on V. 
Note simultaneously that on (r) also. 

(4) Repeat 3 for ten or twelve different voltages on V, rising 
by about equal increments to the maximum, and tabulate as 
follows— 

Name . . . Date . . . 

H.T. Voltmeter tested: No. . . . Type . . ' Range . . . Made by . . . 

Whole resistance Rab = . . . Ohms. Fraction used itac = . . . Ohms. Ratio = . . . 


V. 

Reading on v. 

True voltage 

Rah 

^1 = ^ »'l 

% Error of 
meter tested 

loo(r-Fi). 

Mean 

V. 

Corrected Tj 

Error. 







(5) Plot a calibration curve for the high tension voltmeter 
having values of V as ordinates and true volts as abscissfe. 

Inferences. —Enumerate what you consider to be the advan¬ 
tages and disadvantages of electrostatic voltmeters. 


# 














ELEGTRIGAL ENGINEERING TESTING 


29 


(13) Complete Test of both Direct and Alter¬ 
nating Current Ammeters and Voltmeters 
for the various sources of Errors. 

Introduction, —The principle involved in the action of any type of 
ammeter or voltmeter will come under one of the following heads— 

(1) Heating effect of the current or P.D. to be measured.' 

(2) Electrostatic effect of attraction or repulsion between fixed 
and movable conducting surfaces, close to, but insulated from one 
another, when electrified to opposite potentials. 

(3) Electro-magnetic effect of the current in a coil of wire on 
iron or vice versd. 

(4) Electro-dynamic action of the current in one part of circuit 
on the same current in another part of that circuit, causing 
electro-dynamic attraction or repulsion between the two. 

There are briefly eight principal sources of error to which 
ammeters and voltmeters in general are liable, namely— 

{a) Error in the calibration owing to the standards employed 
in the two cases being different. 

(6) Error through a partial demagnetization of the permanent 
field, causing an alteration in the sensibility, in the case of per¬ 
manent steel magnet instruments. 

_ t 

(c) Error caused by the sensibility of the instrument being 
temporarily altered by external magnetic influence. 

(cZ) Error due to a current producing a different deflection 
depending on the magnitude of the current previously measured 
compared with the one in use at the time. 

(e) Error due to the instrument giving a different scale reading 
for different directions of the same current. 

(/) Error in the case of voltmeters due to alteration of the 
resistance of the instrument caused by change of temperature, 
whether from the room or passage of the current. 

(^) Error in alternating current instrument due to alteration 
of frequency. 

(Ji) Error due to friction at the pivots in all classes. 

Error {a) applies to all four classes of instruments, and cannot 
very well be remedied except by re-calibration. 

It is clear that classes 1 and 2, being entirely non-magnetic, 


30 


ELEGTBIGAL ENGINEERING TESTING 


may be dismissed as not being liable to magnetic errors. Class 
3 , however, and 4, which latter type may or may not contain iron, 
are liable to large errors arising in the case of direct currents 
from the retentivity of the iron used (error d), or magnetic 
hysteresis, and from the proximity of, and the external magnetic 
effect of currents in neighbouring wires and of magnets (error c). 
In the case of alternating currents from hysteresis, eddy currents 
in the metal work about the instrument re-acting on the coil, and 
change of frequency (error g) in the alternating current. 



Fia. 14. 



In this latter class of work the instruments should contain 
no iron at all, and very few metal fittings. In good direct current 
instruments where iron is a necessity, it should be veo'g soft^ and 
well laminated, and there should not be much of it. 

Apparatus. —Ammeter or voltmeter M to be tested; suitable 
variable rheostat R, which in the case of the ammeter test must 
be capable of carrying the maximum current required by J/, and 
in the case of the voltmeter test must have a large resistance so 
as to be quite comparable with that of M; switch, or key E; 
source of electrical supply E, whether direct, or alternating 
current; standard accurately calibrated ammeter or voltmeter Ms 
which must not contain any iron, and preferably no metal fixings. 

N.B.—The standard Ms may be either a Kelvin standard 
balance (p. 546); Siemens electro-dynamometer (p. 577); Cardew, 
or electrostatic voltmeter (p, 562), preferably the latter, which not 
only is non-magnetic, but also has no temperature error. See 
footnote, p. 20, 

Observations. —(1) If an ammeter is being tested, connect up as 
in Fig. 14 (I), but if it is a voltmeter, then as in Fig. 14 (II). Care- 


















ELECmiCAL ENGINEERING TESTING 


31 


fully adjust the pointers of M and Ms to zero if they require it, 
levelling them also when necessary so that all moving parts can 
move quite freely. Place M and Ms at some distance apart so 
that there can be no possibility of one affecting the other. Also 
run the connecting leads close together so that their magnetic 
effect due to the current in them may not affect the instruments. 

(2) Error due to External Magnetic Effect. (A.) 

With R at its maximum, close S, and obtain about ^ full scale 
reading on M. Note the corresponding reading of Ms, which 
must be kept quite constant and steady when no magnet is near. 
Now move a powerful permanent magnet in Si plane perpendicular 
to the axis about which the moving system of M oscillates and 
passing through its centre, the axis of the magnet pointing to M 
always, and its pole nearest to M being moved so as to be always 
at about 12" say from M. Note the alteration (if any) of the 
reading of M. 

(3) Repeat 2 for a full scale deflection on M. 

(4) Repeat the last part of 2, pertaining to the motion of the 
magnet, with no current flowing, i. e. N open, and tabulate your 
results as shown in the table below. 

N.B.—The magnet must not be allowed to affect Ms in any way, 
and the latter must be far enough away to ensure that this is the case. 

If M is an alternating current instrument, and an alternating 
supply is being used, the frequency vaojmiddriQdi constant in 

2 and 3 above, as well as the reading of Ms in the particular test. 

(5) Error due to Retentivity or Residual Magnetism. (B.) 

With R a maximum, close S, and carefully take a gradually 
increasing set of about ten simultaneous readings on M and Ms 
from the lowest to full scale, by gradually diminishing R, gently 
tapping the instruments to eliminate any pivot friction. 

(6) Repeat for a similarly obtained decreasing set, the same 
scale reading of the standard Ms being obtained when descending 
as was obtained on ascending. Tabulate your results as indicated 
in the following table. 

N.B.—This test of course only applies to direct current instru¬ 
ments, and the error in question may amount to over 20%. 


32 ELECTBIGAL ENGINEERING TESTING 

standard instrument used: Type . . . No. . . . Maker . . . Constants . . . 

Instrument tested: Type . . . No. . . . Maker . . . 


A. External Magnetism. 

B. Retentivity or Residual Magnetism. 

Position of 
influencing 
magnet. 

Reading on 

Reading on 

% 

Error. 

Frequency 
(constant) 
if alternating 
current 
used. 

Ms. 

M. 

Ms 

ascending 

and 

descending. 

M 

ascending. 

M 

descending. 









(7) Error due to variation op frequency ” with 
Alternating Current instruments. (C.) 

Close S, and adjust R to give some convenient scale deflection 
on M and 3fs, wliich latter must be kept constant by means of R. 
Now vary the frequency, by altering the speed of the alternator 
if this is under control, from the smallest-to the greatest possible 
so as to obtain about ten different values, and note the simul¬ 
taneous readings on 31 and 3fs at each. 

(8) Error due to Eddy Currents in metal fixings with 

Alternating Currents. (D.) 

If either 31 or 3Is possesses a moving coil, the terminals of 
which can be got at to send a current through this coil only of 
the instrument, as in either a Kelvin balance, Siemens dynamo¬ 
meter, or Parr direct reading dynamometer instruments. Pro¬ 
ceed as follows—Adjust the pointer of this instrument carefully 
to zero, and send the maximum alternating current through this 
moving coil alone, noting whether it deflects. If it does, eddy 
currents are being set up in the metal fixings and re-act on the 
moving coil causing it to deflect. 

(9) Repeat 8 for the same current at different frequencies, and 
tabulate your results as in the following table— 


standard instrument used: Type . . . No. . . . Maker . . . Constant= . . . 

Instrument tested: Type . . . No. . . . Maker . . . 


C. Effect of Frequency. 

D. Eddy Currents, 

Frequency 

CO per sec. 

Reading on 

Frequency 
c/) per sec. 

Reading on 

Ms. M. 

Ms. M. 









































ELECTRICAL ENGINEERING TESTING 


33 


(10) Error due to Eeversal of Current through the 

INSTRUMENT. (E.) 

Connect up as shown in Fig. 14, but instead of connecting 31 
directly in series as shown, join it up now to the circuit through 
a reversing switch or key, so that the current through it may be 
reversed in direction though that in the rest of the circuit and 
therefore tlirough 3Is is still unidirectional. 

With R large, close jS, and obtain say ^ full scale deflection 
on 31, noting that on 3Is which must be constant; now reverse* 
current in 31 and again note its value for the same one as before 
on 3fs. Next re-reverse and note it again. 

(11) Eepeat this operation for about 5 scale readings on 31 
up to the maximum at roughly equal intervals. 

N.B.—This test of course only applies to direct current instru¬ 
ments. Tabulate as follows— 


standard instrument used : Type ... No. . . . Maker . . . Constant = . . 

Instrument tested: Type . . . No. . ■ . Maker . . . 


E. Effect of Current Reversal. 

F. Heating Effect. 

Reading of 

Time of 

Running Hours. 

Reading on 

Ma 

(constant). 

M direction of current. 

Ms. 

M. 

<- 

Reversed. 

—> 

Re-reversed. 









(12) Error in Voltmeters (only) due to Heating op Coils 

BY PASSAGE OF CURRENT. (E.) 

Close S, and adjust R to obtain about J scale reading on 31, 
note the corresponding reading on 3Is which must be an electro¬ 
static voltmeter. Maintain 3Is constant for, say, quarter of an 
hour and again read 31. 

(13) Eepeat 12 for a full scale reading on 31, and tabulate as 
before. 

N.B.—The -error in voltmeters due to change in the tempera¬ 
ture of the room is readily calculable when the latter is obtained 
by a thermometer, 

D 





















34 


ELECTRICAL ENGINEERING TESTING 


(14) Plot the following curves for tests— 

B. Having readings on 31 as ordinates, and 3fs as abscissae 
for both ascending and descending curves. 

C. Having readings on 31 as ordinates and frequency in c/j per 
sec. as abscissce. 

D. Having readings on 31 as ordinates and frequency in c/> per 
sec. as abscissae. 

Inferences. —State very clearly and concisely what you can 
infer from the results of your observations. 


(14) Calibration of a Wattmeter by Compari¬ 
son with a Kelvin Composite Balance 
used as a Wattmeter. 

Introduction. —The following is a convenient method of 
calibrating a Wattmeter by means of direct currents, using a 
Kelvin composite balance as the standard Wattmeter with 
which to compare the one to be tested. The construction of 
the balance is detailed on p. 55':!, where the mode of using it 
as a Wattmeter is also given, and it will merely suffice to say 
here that it is used very similarly to the Hekto-ampere meter, 
the only difference being that as a Wattmeter, the fine wire 
movable coils (only) are placed in series with an extra anti- 
inductive resistance across the mains supplying the power 
measured by both Wattmeters. It may here be noted that it 
is not necessary for the current through the thick winding and 
the pressure across the thin coils to be developed by one and 
the same source. For since the Wattmeter deflection is oc to 
the products of the currents flowing through the two coils, 
clearly these may come from two totally different sources. In 
fact it is distinctly preferable to have them separate when 
possible, for then the variations of the main current will not 
affect the constancy of the pressure on the fine coils. 

This same test serves to determine the “ constant ” (AV say) 
of the Wattmeter, or in other words the number by which the 
scale reading must be multiplied so as to obtain the power in 
Watts. 

The following reasoning will no doubt render this clearer. 


ELECTRICAL ENGINEERING TESTING 


35 


Assuming the general principle and construction of, suppose, 
a Siemens Wattmeter to be understood. Let 0 and c = the 
currents flowing through the fixed thick- and movable thin-wire 
coils respectively when a deflection of the torsion head and its 
pointer on the scale is Z)° or divisions. Then the force acting 
between the coils is oc G x. c, but (c) oc to the pressure V at the 
terminals. Hence the deflecting couple acting between the coils 
cc (7 X F oc Watts. Now when the index is brought back to 
0 again by turning the torsion head, thereby twisting up the 
spring and introducing the control, we have—torsion of spring 
oc Watts oc CV', but the force of torsion is x to angle of 
torsion of such a sprint, 

D az CV 

or KD = CV — Watts measured and causing a deflection D, 
where K is the “ constant ” of the Wattmeter tested. It may 
be found that K is not perfectly constant throughout the whole 
scale. In this case the Watts should be obtained from a calibration 
curve rather than by the product KD. 



Apparatus. —Kelvin composite balance {K.B.) (p. 554), with its 
anti-inductive resistance r (p. 553); switch S; variable power¬ 
absorbing resistance R (p. 606); accurate voltmeter V (preferably 
electrostatic); main current battery B; Wattmeter (IF) to be 
calibrated; [pressure battery b and adjustable resistance if 
available (Fig. 16)]; adjusting rheostat R^ (p. 597). 

Observations. —(1) Connect up either as in Fig. 15 or 16, and 
in the present test assume the latter for actual experiment, and 
make quite certain that the connections are as indicated in Fig. 16. 

(2) Carefully level the instruments that require it, adjusting 
their pointers to zero, and if IF has a suspended coil see that this 
is quite free to move. 

























36 


ELEGTIUGAL ENGINEERING TESTING 


Note.—Care should be taken to run the “leading in” and 
“ out ” wires carrying the main current to IF, and in the rest 
of the circuit close together or twisted in order that the cur¬ 
rents flowing in them shall exert no magnetic influence on the 
instruments. 

(3) Turn the switch on the balance to “ Watls ” so as to place 
the movable fine wire coils across the small terminals. Observe 
whether (r) is numbered the same as, and therefore belongs 
to the balance in use, and make quite certain that the correct 
resistance is being used in (r) (p. 553). 



Fig. 16. 

(4) Adjust the balance and its sensibility by using the proper 
weights as given in the table of constants (p. 558), so that the 
maximum Watts to be measured on IF would give, as nearly as 
possible, a full scale reading on K.B. 

(5) With fairly large, close and adjust the voltage as 
read off on F to the desired amount by altering and then 
maintain this voltage constant, observing that it is so before 
taking every reading. 

(6) A being fairly large, close S, and alter R so as to obtain 
about -iVtli of the maximum “Scale reading on IF. Note simul¬ 
taneously the reading on IF and position (d) of the slider of K.B. 

(7) Repeat 6 for some ten different deflections on IF (by 
varying R) rising by about equal increments to the maximum, 
the pressure remaining constant all the time. 

(8) Repeat obs. 7 for a similar set of decreasing readings 
on IF, and tabulate your results as follows— 































ELECTRICAL ENGINEERING TESTING 


37 


Navk . . . Datb . . . 

Oimf osite Balanre: No, , . . Constants weil Kjj . . , Teinjierature . . . °C 

M attmeter tested: No. . . . Maker . . . Range . . . Maker’s Constant . . • 


Slider 

True 

Reading on JF. 

Constant 

% Error of 
Wattmeter 

Mean 
% Error. 

Reading 

Watts 

Ascending 

Descending 



A 

D. 

D. 

" nic-JiuJ) 









(9) Plot a “calibration” curve for the Wattmeter tested having 
values of D as ordinates and true Watts as abscisste. 

Inferences. —What can be inferred from the results of your 
test? Are Wattmeters subject to any sources of error, and if so, 
how can they be minimized or got rid of ? 

(15) Calibration of a Wattmeter by Com¬ 
parison with a Standard Ammeter and 
Voltmeter. 

Introduction. —The following method of calibration by direct 
currents entails the use of an accurately calibrated standard 
ammeter and standard voltmeter. These may be either Kelvin 
balances or ordinary instruments which have recently been 
carefully compared with accurate standards, and a record of 
the calibration curves of which are obtainable. 

It should be remembered that the constant of a Wattmeter 
obtained with direct currents will only be true for alternating 
currents providing the self-induction of the fine wire moving 
coil or its circuit is practically zero or very nearly so. In other 
words, the instrument must contain no iron and also be very 
nearly “ non-inductive.” This is a matter of great importance, 
for AVattmeters are in most cases only required to measure power 
in alternating current circuits. 

Apparatus. —Standard ammeter (^1) and voltmeter (F); Watt¬ 
meter (IF) to be calibrated with its anti-inductive resistance 
(r) if there is one; battery of secondary cells 7?; switch S\ 
suitable resistance R for absorbing power (p. 606), which must 
be non-inductive if alternating currents are employed; carbon 
rheostat {Rh) (p. 597). 

Observations. —(1) Connect up as shown. Carefully level all 
the instruments, adjusting their pointers to zero, and see that 

















38 


ELEGTIilGAL ENGINEERING TESTING 


the swing coil of the Wattmeter is quite free to move. Care 
should be taken to run the “ leading in ” and “ out ” wires 
carrying the main current to the Wattmeter, close together or 



twisted. Also the main wires of the rest of the circuit close 
together in order that the current flowing in them shall exert 
magnetic influence on any of the instruments. 

(2) E being at its maximum value, close S, and adjust E so 
as to obtain about ^V^h of the full load current through W, the 
pressure being maintained at standard voltage by varying the 
carbon rheostat (Eh). Note the readings of all the instruments. 

(3) Repeat 2 for about ten different readings on W rising by 
about equal increments to the maximum current allowable, and 
calculate for each the percentage error of the Wattmeter and the 
mean. Tabulate as follows— 


Namb . Date . . . 

Non-inductive Wattmeter : No. . . . Maker . . . Temperature = . . . ® C. 


True Volts. 

V. 

True Amps. 

A. 

True Watts. 
W=AV. 

Wattmeter 

Reading. 

D. 

Wattmeter 

Constant. 

D 

Percentage 

Error 

of 

Wattmeter. 

Mean 

Error. 









(4) Plot a curve having values of (IK) as abscissje and the 
corresponding Wattmeter readings (D) as ordinates. 


(i6) Calibration of a Wattmeter with Alter¬ 
nating Currents. (Three-Voltmeter Method.) 

Introduction. —Wattmeters form a class of measuring instru¬ 
ment the chief application of which consists in measuring 


































ELECTRICAL ENGINEERING TESTING 


39 


accurately the power taken up in alternating current circuits. 
The great value of a Wattmeter in such measurements practi¬ 
cally disappears with direct currents as the individual factors of 
power, namely “volts” and “amperes,” are usually here required, 
and in addition the product of the two can easily be obtained 
and at once gives the “ true power.” With alternating currents, 
however, this last remark is not true, and herein lies the great 
value of the properly constructed Wattmeter, in that it measures 
the true power in such a circuit. For it to be capable of doing 
this, however, it must be carefully constructed, and there must 
be no iron and preferably no other metal work near the coils. 
Wattmeters when used on alternating current circuits are liable 
to the following sources of error: (a) owing to the fine wire 
coil possessing some self-induction and consequently impedence, 
the current in it is not able to rise to the same maximum 
strength which it would do for a direct P.D. of similar magni¬ 
tude ; (6) this impedence causes a lag in phase of the current in 
the fine wire coil behind the P.D. across which it is placed. 

(c) A third source of error common also to all voltmeters, and 
occurring both with direct and alternating currents, is that due 
to the alteration of the resistance of the fine wire coil due to 
change of temperature, and which can be minimized in the 
manner described later on. From the preceding remarks it will 
therefore be evident that w^hen a so-called “ Non-inductive Watt¬ 
meter ” is calibrated with direct currents (which is usually the 
case) its “ constant ” so obtained will not be correct for alter¬ 
nating currents. The instrument will also read differently for 
variation of the “ frequency ” of the current even though the 
actual power being measured remains the same. Thus a Watt¬ 
meter may with advantage be calibrated with alternating currents 
on a circuit having the same “constants,” namely voltage, 
frequency and “ wave form,” etc., as that in which it is 
eventually desired to measure the power. The calibration can 
be performed by what is commonly known as the 3-voltmeter 
method of measuring power in alternating current inductive 
circuits, and by it the ^Hrue power'' may be obtained with 
almost any degree of accuracy desired by using an accurately 
calibrated voltmeter and by repeating the observation two or 
three times, noting the mean. It has the advantage that only 


40 


ELECTRICAL ENGINEERING TESTING 


one alternating current voltmeter is required, though three 
similar ones may be used if available. 

Apparatus. —Alternator D and its exciting circuit (not shown) 

under rheostatic control or 
other convenient source of 
alternating current supply ; 
inductive portion EQ of the 
circuit in series with a strictly 
non-inductive portion QR ; two 
2-way keys k^, k^ (p- 587) ; Car- 
dew or low-reading electrostatic 
voltmeter V accurately cali¬ 
brated ; main switch S ; Watt¬ 
meter W to be calibrated; 
alternating current ammeter to 
indicate the current merely 
for reference only. 

Note. —The resistances of 
PQ and QR should both bo 
fairly small compared with that of the voltmeter V. 

Observations. —(1) Connect up as in Fig. 18, and adjust the 
pointers of all the instruments to zero, levelling such as need it. 
See that all lubricators in use feed properly, and then start D. 

(2) Adjust the speed of D so as to obtain the desired 

“ frequency,” say 100 per sec., at the same time vaiying the 
excitation to get the proper voltage, suppose 100 volts across PR. 
Adjust the resistance of PR so as to pass about y^j-th of the full 
load current (necessary to give a full scale reading on IF) through 
W. Then the speed and voltage being constant, note the reading 
on A, W, and in quick succession the voltages and V 

across PR, PQ, and QR respectively by moving and k^ simul¬ 
taneously. 

(3) Repeat 2 for about ten different currents rising by about 
equal increments to the maximum allowable. 

(4) Calculate the power absorbed in PR from the relation 

= 1^2^) Watts, 

where r is the ohmic resistance of QR. 

Tf r is unknown or liable to be altered by the heating effect 





















ELECTRICAL ENGINEERING TESTING 


41 


V 

of the current, its value 0' = —^ may be substituted in the abo^^e 

relation. If the current and voltage are ’sine functions, 
_ 

* 

(5) Repeat 2—4 for a different frequency, say 60 per sec., to 
see whether the Wattmeter “ constant” (K) alters, and tabulate 
vour results as follows— 


cos. 6 


Name . . . Date . . . 

Wattmeter tested: No. . . . Maker . . . Range . . . Temperature . . , 


Speed 

rp.m. 

Fre¬ 

quency 

per sec. 

Power 

Factor 

IFj/ 

Angle 

of 

Lag 

e° 

r Ohms 
or Fa/ 

Ia 

Current 
in Amps. 

A. 

Volts. 

Power. 

Wattmeter. 

Error. 

F 

Vi 


Apparent 

AF. 

True 

)Fi 

Reading 

d. 

Constant 

% 

Mean. 




1 










Note.—Errors made in measuring the voltages V, Fj and Eg 
or in the graduation of the voltmeter scale will have least effect 
on the results when — Eg* the formula in 4 is used with 
the substituted value of (r), this latter may consist of glow 
lamps, as the resistance may vary with the different mean current 
strengths. 

(6) Plot a calibration curve for the Wattmeter tested, having 
values of deflection d as ordinates and true power TFj as 
abscissa?. 

Inference. —Prove the formula given in 4 and state any 
assumption made in obtaining it. What inferences can you 
draw from the results of your test? and explain why the resist¬ 
ance of the voltmeter V should be large compared with either 
FQ or QR. 

(17) Calibration of a High Tension Watt¬ 
meter. (By Ohm’s Law, using an auxiliary 
transformer.) 

Introduction. —It is not always possible in actual practice and 
testing work to avoid the use of a Wattmeter on a high tension 
circuit, as for instance would be the case in measuring the 
efficiency of a high tension transformer run off the terminals 
of a high tension alternator. The W^attmeter in such a case 
should be a specially arranged one for the following reasons— 


































42 


ELEGTRIGAL ENGINEERING TESTING 


(1) Owing to the high pressure in the fine wire moving coil 
circuit, an extremely high non-inductive resistance, capable of 
standing the full pressure across its terminals, would otherwise 
have to be put in series with the fine wire swing coil if an 
ordinary Wattmeter was employed. 

(2) Owing to the difficulty in obtaining the above resistance. 

(3) The risk entailed in handling such an instrument, and of 
the breakdown of the insulation of the whole arrangement under 
the high pressure. 

The best arrangement of a high tension Wattmeter, and which 
gets over these difiiculties, is that shown symbolically in Fig. 19, 
together with the connections for its calibration. 

The Wattmeter W consists of an ordinary Siemens electro¬ 
dynamometer ; the mer¬ 
cury cups, forming the 
terminals of the swing 
coil, are connected to 
a separate pair of ter¬ 
minals by the side of 
the other pair formiug 
those of the fixed 
coil. There is thus no 
electrical connection 
between the fixed and 
moving coils. These 
^ Fia. 19. latter are connected 

to the low pressure 
coil of a small auxiliary transformer T. The high tension side 
of T is placed across the high pressure mains {m.m.), hence the 
moving coil of W passes a current which depends on the E.M.F. 
of m.m., and at the same time there is no fear from a breakdown 
of insulation since both coils of W are passing ordinary currents. 
The actual current which T sends through the swing coil of W 
may be as small as convenient. 

Apparatus.—High tension Wattmeter W to be calibrated 
arranged as mentioned above, with its fixed and movable coils 
separate. Small auxiliary (H.T.) transformer Ty high and 
low tension electrostatic voltmeters V and v respectively; 
strictly non-inductive resistance ACB capable of being placed 
















ELECTRICAL ENGINEERING TESTING 


43 


across the (H.T.) mains m.m., and of carrying enough current 
at that pressure to enable a considerable scale deflection to 
be obtained on W. A part AG of the whole resistance AB 
should have such a value (?*) and be of such a carrying capacity 
as not to be heated and changed by the current through ACB 
and as will have a P.D. across its terminals capable of being 
read on v. 

Note. —As a precaution, india-rubber gloves must be worn, 
and an india-rubber mat provided to stand on. 

Observation. —(1) Connect up as in Fig. 19, and adjust the 
instruments carefully to zero. 

(2) Close switch {S) and adjust F to read the desired amount 
which IF has to deal with on future occasions. Note the read¬ 
ings of F, r, and IF, and tabulate as follows— 


Name . . . Date . , . 

Wattmeter tested: No. . . . Maker . . . Range . . . Temperature . , . 


Noii-Iuductive Resistances. 

Voltages. 

Current 
through ACB 

A=':- 

r 

True mean 
Watts in ACB 
Wi=VA 

Reading on 
Wattmeter 

<^w 

Constant of 
Wattmeter 

R. 

r. 

V. 

V. 










Inferences. —Is the method liable to any sources of error ? and 
if so, state them. 


(i8) Calibration of an Electricity Meter 
(on Constant Supply). 

Introduction. —An electricity meter, which performs the same 
kind of oflice to a consumer of electrical energy that a gas-meter 
does to one using ordinary gas, is an electrical instrument that 
requires carefully calibrating or standardizing at some time or 
another. There are a great number of different forms of electricity 
meters, but they all come under one or other of four main classes, 
namely— Electrolytic, Thermal, Motor, Clocks affected. It 
is not, however, our intention to dilate on these further as their 
theory and description comes under the scope of the ordinary text¬ 
book, but there are some points in general which may be remarked. 
Practically all meters measure one or other of two things, namely. 



















44 


ELEOmiGAL ENGINEERING TESTING 


(a) Ampere-hours, when they are called quantity- or Coulomb- 
meters, (6) Watt-hours, when they are termed Energy- ov Joule-meters. 
In most cases, though by no means all, meters are graduated and 
read directly in the official “ Board of Trade Unit ” (1000 Watt- 
hours). It must not, however, be supposed that because a meter 
reads Board of Trade units on its dials it is a true energy 
meter in the real sense of the word, i. e. a meter containing a 
current and pressure coil acting on one another in a suitable 
manner, for if the pressure is pre-assumed and taken as being 
constant it is an easy matter to graduate and calibrate the dials 
of a Coulomb-meter to read directly in B.O.T. units. This it may 
be remarked is generally done now. 

Apparatus. —-Accurately calibrated ammeter^,and voltmeter V j 
secondary battery B, or steady source of supply; switch S ; power 

absorbing resistance L, or 
bank of lamps (p. 598); 
rheostat R (p. 597), which 
might be required for ad¬ 
justing the pressure on the 
mains j meter M to be 
tested. When possible, it is 
best and most economical in 
power used, to employ two 
distinct circuits, one giving the necessary voltage for the fine 
wire coil of J/, if there is one, the other the necessary current 
through the current circuit of M. These two sources must be 
secondary batteries if possible so as to be quite constant. 

Note. —If the meter to be tested is an alternating current one, 
then A and F should be alternating current instruments, such as 
a Siemens electro-dynamometer and electrostatic voltmeter re¬ 
spectively, or the author’s instruments. R also should be non- 
inductive, and L may preferably consist of a bank of lamps (p. 598) 
or other non-inductive resistance. If an accurately calibrated non- 
inductive Wattmeter is available, then the true power given to L 
can at once be obtained irrespective of the nature of R and L and 
also without using A. In such cases the meter should be tested at 
different frequencies and on inductive loads. 

Observations. —(1) Fix up the meter in a position as nearly 
vertical as possible and connect it in circuit so as to register the 

















ELEGTRIGAL ENGINEERING TESTING 


45 


quantity (amp.-hours) or energy (Watt-hours) as the case may be, 
given to X. 

(2) If the meter is intended for use on 100 or 200 volt circuits, 
close S and vary L so as to absorb the full load current of the 
meter, and adjust R so that F reads the required voltage. 

(3) Open S and take the dial readings of the meter. 

(4) At a known tabulated instant, switch on, and keep the 
current (A) and pressure (F) constant for about -Ahour by alter¬ 
ing R and E if necessary. Then switch off at a noted instant 
and take the dial readings again. 

(5) Kepeat 2, 3, and 4 for and full load currents 

through the meter, and tabulate as follows— 


Name . . . Date . . . 

Meter tested : T> pe . . Maker . . . No. . . . 

Amps, (full load)= . . . Voltage (if any) . . . Dial reading in . . . 


Meter reading 

Amount 
Regi .stored 

Amps. 

A. 

Volts. 

V. 

Time. 

T (Hours). 

True amp. 
or Watt- 
hours. 

% error o' 
Meter. 

Start $j. End Qo. 









Inferences. —State the chief conditions which a meter of the 
above type should fulfil. 


(19) Complete Test of an Electricity Meter. 

Introduction. —In order to completely test an electricity meter 
in the way that would be advisable with any new type of instru¬ 
ment, three or four additional tests other than the preceding one 
should be carried out and are as follows :— 

(a) Starting 'power of the Meter. — Obtained by carefully 
measuring the least current or Watts that will just cause the 
meter to start. It is obvious that such should not exceed the 
amount used up in the smallest lamp employed. 

(h) Effect of external Magnetism .—Which can be investigated 
by sending a steady current or number of Watts through the 
meter according to what it measures, and then bringing a strong 
magnet into a number of different positions about the outside of 
the meter. The record of the instrument taken over a sufficient 



















46 


ELECTRICAL ENGINEERING TESTING 


period for each position of the external magnet should remain 
unaltered. 

(c) Power absorbed in the Shunt-coil. —Some meters possess a 
fine shunt coil of considerable resistance for the purpose of pro¬ 
viding the instrument with a sufficient magnetic field to enable it 

O O 

to start with a very small current. The amount of power absorbed 
in this coil, if there is one, should be carefully measured and the 
cost of it per annum calculated on the basis of say ^d. to 4c?. per 
B.T.U. if the supplier pays for it as he should. It should also be 
observed whether this coil is across the lamp or supply side of the 
meter in order to see if the consumer or supplier pays for the 
power so wasted, for in the aggregate the cost of this may amount 
to a considerable sum in the course of the year. 

(d) Gradual deterioration of ivorking parts. —This is most 
important, but can only be determined by a “ time test ” extend¬ 
ing over a considerable period amounting to months. 

Thus to furnish a true and accurate report on an electricity 
meter, investigations (a — d) should be undertaken, and in addition 
the test immediately preceding them. 


(20) Measurement of a Resistance heated by 

an Electric Current. 

Introduction. —When a resistance is heated by the passage of 
a current its value so heated may or may not be very different 
from that when it is cold. Thus, for example, a resistance com¬ 
posed of the alloys Manganin or Eureka, etc., would alter its 
resistance very little for considerable changes of temperature; 
whereas if made of carbon, the specific resistance of which dimin¬ 
ishes rapidly as the temperature rises, the resistance would be 
very different when hot to what it would be while cold. In 
practice, however, one of two conditions may occur in connection 
with heated resistances, namely, (1) The type and form may be 
such that the temperature does not fall very quickly immediately 
the current is cut off, thus enabling time measurements of resist¬ 
ance to be taken with, say, a Wheatstone Bridge or other suitable 
means, and the resistance hot, at the moment of breaking the 
current, to be obtained graphically by plotting the results; owing, 


ELEGTRIGAL ENGINEERING TESTING 


47 


however, to the difticulty of taking rapid time measurements of 
resistance and the introduction of other errors we shall not con¬ 
sider this method further. (2) The type of resistance may be 
such that the temperature falls very rapidly and far too quickly to 
enable any measurements, as in 1 above, to be taken. Such is 
the case with the filament of an electric incandescent lamp, and in 
order to obtain its resistance (warm), accurately, during the 'pas¬ 
sage of a current^ and which is absolutely necessary, another 
method has to be employed other than that of the Wheatstone 
Bridge, etc. 

Though the results of the present test will disclose the fact, it 
may be mentioned here that the filament resistance of an electric 
glow-lamp when burning normally is very different from that 
when cold. Thus this latter, which can best be obtained by the 
Wheatstone Bridge, would not in any way represent the true 
resistance while the filament is under working conditions. 

We will assume that the resistance in question is of the 
nature of an electric glow-lamp and therefore cools far too 
rapidly to allow of the employment of the first method mentioned. 
For a concrete case suppose that the resistance of the filament of 
an electric incandescent lamp is required at different luminosities, 
i. e. when different currents are passing through it. This resist¬ 
ance will diminish as the current increases, or as the temperature 
increases, owing to the specific resistance of carbon diminishing as 
the temperature rises. This property of carbon, in having a 
negative coefficient of variation of resistance with temperature, 
should be remembered as compared to the same property of all 
the metals and nearly all the alloys, of which Manganin ” may 
be cited as an exception in having a negative coefficient. The 
present method is a direct application of Ohm’s Law, and consists 
in measuring the current through the lamp and the voltage across 
its terminals. There are, however, some precautions which have 
to be adopted in order to obtain the true voltage and current^ and 
these we may now point out in connection with the two arrange¬ 
ments possible with this method. In each of the cases I. and II. 
( V) represents the voltmeter, {A) the ammeter, and R the glow- 
lamp or other resistance to be measured while hot. The rest of 
the main circuit is omitted for simplicity, but may comprise a 
suitable secondary l)attery and a rheostat foi varying the current 


48 


ELECTRICAL ENGINEERING TESTING 


through R. In Case I. the voltmeter V is connected directly to 
the terminals of R, hence the ammeter A will measure the sum of 





"uWWl; 

R 

(I) 



the currents through V and R together. But the method requires 
the actual current through R only, which is found as follows—, 
Let ii^=the true resistance of the voltmeter in ohms. 

Then-i^= the true current flowing through it in amperes, 
Ry 

Y 

whence the actual current through R = A — — amperes and the 

1\> y 


resistance of R (hot) =- ^ 

A - ohms. 

-/i y" 

To see the magnitude of the error caused by neglecting the 
correction for the voltmeter current, suppose F= 100 volts, Ry = 
10,000 ohms and A reads 0‘61 ampere. Then without correction: 


R (hot) =-^= = 163-93 ohms, and with correction R (hot) 

^ ^ A 0-61 ' ^ ^ 

V _ 100 _ 100 , ^ , 

V 0-61 - 0-60 “ 

In other words the error made in the resistance, neglecting the 
correction, = 1-64%. It should, however, be remembered that 
the average commercial voltmeter has a resistance much less than 
10,000 ohms, and hence the above error would be much greater 
when using such. If Ry — Infinity then no correction is neces¬ 


sary; for, the resistance of R (hot) then 



_F 

A 


ohms or the voltmeter passes no current. 






















ELEGTRIGAL ENGINEERING TESTING 


49 


Sucli is the case with any electrostatic instrument such as the 
Kelvin multicellular voltmeter, the resistance of which is prac¬ 
tically infinite. It is therefore a good one to use for the purpose. 

In Case II. the voltmeter is connected across the ammeter and 
resistance combined, and it will therefore measure the su 7 n of the 
voltages across R and A. But the actual voltage across R only, 
is required by the method, and this is obtained as follows— 

Let Rj, = the true resistances of the ammeter in ohms. 

Then = the true voltage across the ammeter terminals, 

whence, the actual voltage across R — V — ARj^ volts and the 

resistance of R (hot) = —— - --- ohms. 


To see the error caused by neglecting the voltage lost in the 
ammeter. Let F read 100 volts and A read 0*6 ampere, assume 
R^ = OT ohm, which is about the value for an instrument reading 
such small currents. Then we have 


without correction i? (hot) = ^ = 


100 


and with correction R (hot) 


0-6 

y-AR^ 


= 166’66 ohms. 

100-0*06 99 94 


•6 


= 166-57"- 

Or the error made in not allowing for the loss of voltage in A 
is 0*054%. Thus, although the resistance of F can easily be 
measured on a Wheatstone Bridge and that of A either by the 
bridge or by the “fall of Potential’’ method {vide p. 84), 
w’hen these cannot be readily obtained, we see that Case II. Avill 
give the best results and the freest of the two from error by 
neglecting any corrections. 

Apparatus. —Accurate ammeter A and voltmeter F; the 
resistance R or glow-lamp to be measured while hot. The rest 
of the main circuit, if this is not already set up, comprising 
secondary battery, rheostat r, switch s, etc. Arrangements should 
be at hand for measuring the resistance of F and A, viz.—P. 0. 
Bridge, galvanometer, Leclanche cell and standard known 0*1 
ohm resistance, etc. 

Observations. —(1) Connect up as in Case II. Fig. 21, and 
adjust the pointers of F and A to zero. 

(2) Measure the resistance of the ammeter, voltmeter, and 
lamp (cold) by suitable means. 

E 







50 


ELECTRICAL ENGINEERING TESTING 


(3) Take a series of simultaneous readings on A and V for 
different voltages, rising by interv^als of 10% from 0 to within 
about 5% of the normal, and then by steps of 1% to 5% above 
normal voltage 

(4) Repeat (3) above, using a metal filament lamp instead of 
the carbon one, and tabulate thus—- 

Date**. 

Nature of Resistance tested . . . Resistance Cold = . . . Ohms. 

Ammeter ,, . „ 

Voltmeter ,, liy = . , . ,, 


Reading on 
Voltmeter. 

True Volts ( V) 
across Lamp. 

Current through 
Lamp (A) Amps. 

Resistance (hot) 
F- AJU 

- z — Ohms. 

A 

% increase or 
decrease of resist, 
when hot. 







(5) Plot curves for each lamp having values of lamp resistance 
as ordinates and corresponding currents through it and voltages 
across it, both as abscisste. 


(2i) Measurement of the Efficiency and 
Candle Power of Electric Glow Lamps. 

Introduction. —At the present day, when new forms of electric 
incandescent lamps are frequently making their appearance before 
the general public, or otherwise, it becomes of scientific interest 
and often of practical importance to thoroughly test the advan¬ 
tages claimed for these particular forms, and to discover their 
disadvantages. Amongst others, the chief investigations should 
be— 

(a) The eflSciency at and about the normal or rated voltage, 
stamped on the lamp by the makers. This is reckoned in “ Watts 
per candle ” for commercial purposes, though it should more 
correctly be termed its “in-efficiency.” The number of “candles 
per Watt ” more properly denotes the efficiency of a glow lamp 
as a light-emitting source. 

(^) The candle power (C.P.)at the rated voltage and in a given 
direction. 

(y) At what efficiency, the total cost of operating this 
particular form of lamp, is a minimum. 

These three investigations are of considerable moment to the 
user of such lamps, and the results practically decide whether his 
annual expense of electric lighting, using such a form of lamp, 
would be greater or less than with the present lamps in use. It 











ELECTRICAL ENGINEERING TESTING 


51 


may probably be the case that it is impossible to do anything 
with the investigation marked (y), owing to there being insuffi¬ 
cient data to hand, the data required being (1) the cost of energy 
supplied, (2) the cost of lamp, (3) rate of variation of the life 
with Watts per candle, which is the most difficult item of the 



A, B. 


Fig. 22. 

three to obtain. For purposes of discussion, however, as this 
particular question is of considerable interest, we will assume 
that very carefully-made tests give the relation between the life 
in hours of the lamp and the efficiency, or Watts per candle 
used, as shown in curve A, Fig. 22. Then on plotting curves B 
of the same fig., the cost of lamp renewals per hour will give us 
the curve NPR, and the cost of energy per candle-hour will give 
us the straight line OPQ. Summing the ordinates of the two 
curves, we get the third curve NTS convex to the abscissae and 
representing the total cost of the only two sources of expenditure, 
namely, cost of lamps and cost of energy. If now an ordinate 
through the lowest point T of this third curve cuts the abscissie 
in the point D, then OD (in this case 3'8) gives the efficiency, or 
Watts per candle, at which the total cost of operating this form 
of lamp is a minimum for the particular electrical supply taken. 

The two first-named investigations (a and are contained 
in the following relations, which must be determined, viz.— 
the variation of C.P. with (1) amperes, (2) volts, (3) Watts, 
(4) efficiency (Watts per candle), (5) resistance of filament, and 
(6) the cost per candle-hour for energy with efficiency. 










52 


ELEGTRIGAL ENGINEERING TESTING 


Tlio candle power may be obtained by employing some 
convenient form of photometer, such as any one of those 
described on p. 589. We shall, however, here assume the use of a 
Bunsen grease-spot photometer, the carriage of which slides along 
an ordinary straight graduated bank or bench containing the 
standard of light at one end and the glow lamp to be tested at 
the other. If now C = the C.P. of the standard of light, and (d) 
its distance from the grease spot when “ balance,’' in the manner 
to be described later, is obtained, also if Z) = distance between 
standard and lamp to be tested, then the C.P. of this lamp is 


K = G 




Now to facilitate working out the results of the tests, a cali¬ 
bration curve for the photometer bench may be drawn from 
calculations in which values of {d) are abscissee, and the corre- 

spending ones for ^- ■■as ordinates. Thus the ordinates of 


this curve x by the C.P. of the standard will give directly the 
C.P. of the lamp to be tested corresponding to the particular 


distance (cZ). 


The values of 


'D-dV 


d 


for various values of d 


when = 3, 4, 5 and G metres, are given in the Table, p. 651, 

and they will be found to save a great deal of time in working out 
the results of photometric tests in general. Intermediate values 
not given in the table can best be obtained from the curve plotted 
between the numbers pertaining to the value of D used in the 

test and the values of ~ ^ 

V d 

Eeferring to the above formula for calculating the unknown 
C.P., we see that any error made in reading the true position of the 
carriage carrying the “ grease spot ” or other balancing device will 
have minimum effect when K = G or d={D- d), i. e. when this 
carriage is at the midway position between the two sources of light. 
The same kind of thing occurs in the case of measurements of 
resistance by the “Metre Bridge.” To fulfil, however, the 
relation just mentioned, it will in most cases be necessary to 
employ a subsidiary or intermediate standard of light, such as a 
good electric glow lamp, which has itself been very carefully 
standardized by reference to an ordinary smaller standard. 







ELEGTRIGAL ENGINEERING TESTING 


53 ' 

Considerable difficulty may sometimes be experienced in 
balancing on the photometer with the naked eye when different 
sources of light are being compared owing to the difference in 
colour of the lights. In such cases it is of advantage to observe 
the “ sight-box ” containing the Bunsen grease spot or otlfer 
arrangement through coloured glass when balancing; that 
known commercially as “ signal red ” and “ green ” being the best 
for the purpose, and two pieces should be chosen, so that on 
looking through the two together in bright daylight next to no 
light passes through them. Thus on balancing the sight-box by 
observing through each separately, the mean of the readings will 
afford a correction to a certain extent for any difference in colour 
of the two sources of light. 

The principal object gained in using coloured glasses is that 
the eye then observes a less bright surface, and is consequently 
better able to gauge its illumination relatively to the surrounding 
surface. It is a fact that when the eye looks at a very bright 
surface, the pupil of the eye partially contracts, thus causing the 
effect of temporary partial blindness, hence the use of coloured 
glass to prevent this. 

It should be carefully remembered that if the resistance of 
the voltmeter, which measures the pressure, is not very high 
compared with that of the lamp (say exceeding 100 times), 
and the ammeter resistance not very low compared with that of 
the lamp (seldom the case), then corrections must be applied to 
one or other of these instrument readings, in order to obtain 
either the true voltage on the lamp or true amps, through it. 
For such see test on p. 48. 

Apparatus. —Low reading ammeter A with long open scale (p. 
559); high resistance voltmeter V; adjustable fairly high resist¬ 
ance R (Fig. 272) ; secondary battery and photometer bench DD 
complete, containing “Methven screen” 2 C.P. standard (C) 
of light (p. 595), or Fleming standard glow lamp; glow lamp G 
to be tested; and ‘^sight-box” E containing a Bunsen grease 
spot (p. 592), or Flicker photometer head; switch S. 

N.B. _For particulars on the adjustment and use of the 

Methven standard, see Appendix, p. 595. The lamp G to be 
tested may be run as low as will give measurable luminosity, but 
not higher than 5% above its normal voltage 


54 


ELECTRICAL ENGINEERING TESTING 


Observations.—(1) Fix, in a manner most convenient for calcu¬ 
lation, the distance D between standard and lamp to be tested 
(500 cms. say), and adjust the standard to the certified standard 
value of C.P. 

if a standard glow lamp is used instead of C connect it to a 
constant voltage supply through an exactly similar circuit to 
that shown for G, but without an ammeter {A). 

(2) Connect up the glow lamp as indicated in Fig. 23, and 
adjust the pointers of V and A to zero, levelling the instruments 
carefully if necessary. 

(3) Measure the resistance of the lamp filament while cold by 
means of the Wheatstone Bridge, and note its value. 



(4) Close S and adjust R, so as to obtain just measureable 
luminosity on G, then move the carriage carrying the “grease 
spot ” until the whole surface of the latter appears equally 
illuminated. Great care being taken to keep the standard of light 
pro 2 ')erly adjusted all through the tests at its certified value. 

N.B.—The true scale position will be found more accurately by 
taking the mean of the two positions of the carriage when the 
spot is just perceptibly darker and lighter respectively than the 
surrounding paper, or with a Flicker head when the Flicker 
disappears. 

Kote in each case the distance {d) from standard to grease spot 
when the plane of the lamp filament is (a) parallel, (6) perpen¬ 
dicular to the axis of the bench, coloured glass being used in each 
case if necessary. 
































































ELECTRICAL ENGINEERING TESTING 


55 


(5) Repeat 4 in 10% intervals of voltage across the lamp 
terminals up to within about 5% of the normal voltage for the 
lamp tested, and afterwards in 1% intervals to the maximum 
allowable. 

(6) Calculate the C.P. (K) of the lamp at each voltage from the 
relation 

K = G ~ 

where (7 = C.P. of the standard (d) = mean of all the bench 
readings of the grease spot as found in 4 above. Tabulate all 
your results as follows— 

Name . . . Date . . . 

lAimp tested : Make .: . Class . .. Normal Volts . . . Normal C.P. . . Resistance Cold . . . 
Standard of Light: Tyjjo . . . C.P. = . . . . . . 


Current 

U) 

Amps. 

P.D. 

(O 

Volts. 

Power 

(IF) 

Watts. 

Resist¬ 

ance 

(hot) 

(Ohms). 

Plane of Filament and axis 
of Bench. 

Parallel. |j Perpendicular. 
Spot just perceptibly 

Total 

mean 

posi¬ 

tion 

(d) 

C.P. 

A' 

Effici¬ 

ency 

in 

Watts 

]>er 

candle. 

Cost 

pier 

candle- 

hour 

for 

power 

P 

dark 

light 

dark 

light 



1 










(7) Plot the following curves, to the same pair of axes and 
same scale for C.P. {K) on the ordinates, between K and (1) 
amperes, (2) volts, (3) AYatts, (4) resistance, (5) distance {cl), (6) 
elHciency, (7) the cost P. 

(8) Calculate the ratio of the resistance “hot” to that “ cold ” 
for the lamp filament. 

Ifote.—In calculating the cost P at the various C.P.s, assume 
that electrical energy costs ^d. per Board of Trade unit (1000 
Watt-hours). 

Inferences.—State at some length all the inferences which can 
be drawn from the above experimental results and curves. 

(22) Variation of Candle Power with direction 
around an Electric Incandescent Lamp. 

Introduction.—-With the introduction of new designs of electric 
glow lamps at the present day it is of considerable interest and 
often of importance to see the way in which the magnitude of the 
C.P. along a fixed or given direction changes as the lamp is turned 
































56 


ELECTRICAL ENGINEERING TESTING 


through various angles in both horizontal and vertical azimuths. 
The glow lamp to be tested should be capable of being turned in 
any direction about a point which is the centre of the principal 
part of the filament, and, further, this point must be in a line with 
the standard of light and centre of the Bunsen grease spot or 
other “ sight-box.” 

If the lamp thus adjusted is supplied at constant voltage and 
the C.P. measured at different angles in the horizontal plane as 
the lamp is turned completely through the circle, then the mean 
of all these C.P.s gives what is termed the “onea7i horizontal 
C.P” If in addition the C.P. is now measured all round a 
vertical circle, the plane of which successively makes different 
angles with the a,xis of the photometer bench around a horizontal 
plane, then the mean of all the results will give what is termed 
the “ mean sijheilcal G.F.” and it will be found that the ratio 

mean spherical C.P. 
mean horizontal C.P. ~ ^ 

a constant which may be determined in the manner to be described 
presently. 

The variation of C.P. around the lamp, as found in the present 
test, can best be seen by plotting “ polar curves ” for the different 
planes in which the filament is turned, using “ polar co-ordinates.” 
We will now consider briefly the approximate general form and 
method of plotting such curves. 



Fig, 24 I. and II. represent polar curves showing the distribu¬ 
tion of luminosity in a horizontal and vertical plane respectively. 









ELEGTFdGAL ENGINEERING TESTING 


57 


To obtain I. take any point or pole (P), and with it as centre 
describe a circle, the radius of which represents to a suitable scale 
either the normal rated C.P. of the lamp, stamped on it by the 
makers, or else, the mean horizontal C.P. as determined from the 
experimental results. Divide the circle (shown dotted) into 12 
equal divisions of 30® each all round from 0° to 360, and draw a 
radial line from P through each, then setting olf the C.P.s 
measured at the respective R,ngles on these radial lines, the con¬ 
tinuous curve is obtained on suitably joining them. In Fig. 24 
I. the starting-point 0° would bo that position of the lamp when 
the plane of the filament is parallel to the axis of the photometer 
bench. In II. the datum circle is drawn in the same way as 
before (and is shown dotted), and on setting off the C.P. measured 
at the respective angles as the lamp is now turned in a vertical 
•plane., the continuous curve, somewhat of the shape shown, will 
be obtained. This polar curve II. will only represent the dis¬ 
tribution of C.P. in that particular vertical plane which makes 
some noted angle with the above-mentioned zero on the horizontal 
plane or circle. 

Apparatus.— Precisely the same as that mentioned in the pre¬ 
ceding test, with the single exception that the glow lamp is now 
held in a special form of holder capable of turning through known 
angles in horizontal and vertical planes. 

Observations. —(1) Connect up as indicated in Fig. 23, and 
adjust the pointers of the instruments V and A to zero, levelling 
the meters if necessary. 

(2) Fix, in a manner most convenient for calculation, the 
distance D between standard and lamp to be tested (500 cms. say) 
and adjust the standard of light used to the proper C.P., either 
using the gas carburettor or otherwise. 

(3) Close S and adjust R so as to obtain exactly the normal 
rated voltage across the lamp terminals, which must be kept 
perfectly constant throughout the whole set of tests. 

(4) Adjust the lamp and its holder so that the principal part 
of the filament can rotate in a horizontal or vertical plane about 
some fairly definite point, the line joining which to the standard 
passes through the grease spot and is parallel to the photometer 
bench. 

(5) With the voltage at exactly the normal value and the index 


58 


ELEGTBIGAL ENGINEERING TESTING 


at zero on the horizontal scale for the plane of the filament parallel 
to the bench, measure the C.P. every 30® on the horizontal scale 
as the lamp is turned round (vide obs. 4 and G of the last test).. 

(6) With the index at 0° on the last-named scale measure the 
C.P. every 30° on the vertical scale as the lamp is turned round. 

(7) Repeat 6 for the index at 45° and 90° on the horizontal 
scale, and tabulate all your results as follows— 


Name . . . Date . . . 

Lamp tested :— Make . . . Class . . . Normal Volts . . . C.P. . . . Resistance Cold . . . 
Standard of Light, Type . . . C.T. . . . D= , . . 


Horizontal 

scale 

readings. 

Vertical scale readings for different 
horizontal angles. 

Distance 

d 

C.P. 

K 

Mean 

K 

Ratio 

M.8.K. 

0? 

45'> 

90° 

M.ILK. 










(8) Plot the polar curves for horizontal and vertical distribu¬ 
tions in the manner set forth above. The latter for each of the 


angles 0°, 45° and 90° 


(23) Measurement of the Percentage Absorp¬ 
tion of Light by different kinds of Shades 
and Lamp Globes. 

Introduction. —In electric lighting particularly, and also in 
other methods of illumination, it is almost invariably the case 
that the lamp is partly or wholly enclosed in a globe or shade 
which is partly for use in softening the light, as we may express 
it, and partly for ornament. In arc lighting the opalescent globe 
is very generally used, while in the case of electric incandescent 
lighting either the bulbs of the lamps themselves are often made 
of opalescent, ground, frosted, coloured or other translucent forms 
of glass, or separate shades of such material enclose the ordinary 
clear glass bulb of the lamp. In all cases the result is a more 
evenly diffused light and a more uniform illumination, and one 
that is softer, so to speak, and less trying for the eyes. The 
introduction of such shades, however, usually diminishes con¬ 
siderably the outside illumination of the source owing to the 
absorption of light by the shade, but it should be borne in mind 





















ELECTRICAL ENGINEERING TESTING 


59 


that some of the light which is apparently absorbed is actually 
lost by reflection. As, in some cases, so much as 70% of the 
light produced is thus absorbed, it becomes of importance to 
determine the amount in particular cases, for it will be seen to 
materially affect the number of lamps really needed to illuminate 
satisfactorily a room of given area. The present test is arranged 
v/ith the object of doing this, but no account will be taken of loss 
of light through reflection, as the measurements of this and 
absorption separately requires more elaborate methods. 



Apparatus. —Low reading ammeter A with long open scale 
(p. 559); high resistance voltmeter V: adjustable fairly high 
resistance R (p. 603); secondary battery and photometer bench 
DE complete, containing “ Methven screen ” or other standard {C) 
of light; “sight-box’’ B containing a Bunsen grease spot (p- 
592); switch S; and an electric glow lamp G (say of 8 C.P.); 
shades to be tested. 

N.B.—For particulars on the adjustment and use of tlie 
Methven screen standard of light, see Appendix, p. 595. 

Observations.—(1) Fix, in a manner most convenient for cal¬ 
culation, the distance D between standard and glow lamp (500 
cms. say), and adjust the standard to the proper C.P., either using 
the gas carburettor or otherwise. 

(2) Connect up the glow lamp G as indicated in Fig. 25, and 





























































60 


ELEGTEIGAL ENGINEERING TESTING 


adjust the pointers of V and A to zero if necessary and levelling 
them if required. 

(3) Close S and alter E so as to obtain the normal voltage across 
the lamp terminals as read olf on V. Then move the carriage 
carrying the “grease spot” until the whole surface of the latter 
appears equally illuminated, great care being taken to keep the 
standard of light properly adjusted throughout the tests. 

N.B.—The true scale position will be found more accurately 
by taking the mean of the two positions of the carriage when the 
spot is gust 'percei^tihly darker and lighter, respectively, than the 
surrounding paper. In this way the mean distance (cZ) from 
standard to grease spot should be taken when the plane of the 
lamp filament is (a) parallel, (i) at 45°, (c*) perpendicular to the 
axis of the bench, and the volts and current noted at each (which 
must be kept constant). 

(4) Now place a given shade to be tested for absorption over 
this plain glass bulbed lamp and repeat 3 for this and all other 
shades in succession, keeping the lamp voltage quite constant. 

(5) Calculate the C.P. ilC) of the light with and without shades 
from the relation 

K = G ^ candles, 

where G = the candle power of the standard and d — mean of all 
the bench readings of the grease spot as found in 3 above, and 
tabulate your results as follows— 


Name . . . Date . . . 

Glow Lamp ; Normal Volts= . . . Nature of glass bulb=: . . , 

Standard of Light: Type . . . C.r. . . . Total d'stance D= , , , 


Nature 

or 

hind of 
sliade 
tested. 

Volts 

Y. 

Amps. 

A. 

Plane of Filament and axis of Bench 

Total 
mean 
of all 
posi¬ 
tions 
(d). 

C.P. of light. 

Absorp¬ 

tion 

by shade 

100 

Aq 

Parallel. 

At 45°. Perpendicular. 

With¬ 

out 

shade 

With 

shade 

A'l 

Si)ot just perceptibly 

dark 

light 

dark 

light 

dark 

light 













1 


Inferences. —State carefully all you can infer from the results 
of your experiments and point out their bearing on the lighting 
of rooms in general. 






































ELECTRICAL ENGINEERING TESTING 


61 


The Photometry of Electric Arc Lamps, 

General Remarks. —In photometric measurements of the 
present nature, many little difficulties exist which do not af)pear 
in the study of nearly all other sources of light. They arise 
from the fact that, in the first place, the intensity of the light is 
extremely fluctuating and very difficult to maintain constant for 
any appreciable length of time. In the next place, the in¬ 
tensities are usually so large that special standards of light have 
to be employed, and in addition, the general arrangements of the 
arc light source relatively to the photometer and standard have 
to be such that only a known fraction of the light to be 
measured is- balanced against the standard. The chief difficulty, 
however, arises from the great difference between the colour of 
the arc light and that from all other common standards of light. 
So marked is this, both in the case of direct and alternating 
current arcs, that frequently the unpractised eye is unable to 
form anything like an accurate judgment between the amounts 
of illumination on a given surface due to each. Lastly, the arc 
is continually travelling round the carbons, thereby causing wide 
variations in the light emitted along the axis of the photometer 
bench. This effect on the correct readings of the photometei 
‘‘ sijht-hox^’ for given positions can be minimized by taking the 
mean of several readings of the sight-box on the bench for as 
nearly as possible the same values of voltage and current supplied 
to the lamp. 

As is well known there are some characteristic differences 
between continuous (direct) and alternating current arcs. In 
the former, the positive and negative carbons burn away at rates 
approximately in the proportion of 12:7 respectively, which, 
however, may vary from 1 ’2 to 4, according to the quality and 
type of carbons and the voltages and current supplied to the 
arc, the first-named ratio only holding approximately true for 
ordinary lamps with carbons of equal diameter, and using from 
10 to 12 amps, at 45 to 50 volts and run with continuous currents. 
Peculiar to this type of lamp is the formation of a recess or 
as it is commonly called, at the end of the carbon, 
the — assuming a conical pointed shape at the end. 


ELECTRICAL ENGINEERING TESTING 


For lighting purposes the lamp is always connected up and 
suspended so that the carbon is uppermost, and no second 
consideration is necessary to at once see that the distribution of 
light all round the arc (^. e. the spherical distribution) is far 
from being uniform. This determination, together with that of 
the spherical candle power (C.P.) for a given amount of power 
absorbed by the lamp and the regulation of the lamp mechanism, 
amongst other tests, is the object of the following investigation. 

The distribution of intensity requires for its determination 
somewhat special arrangements for enabling the C.P. to be taken 
at different angles to the horizon line. This variation is 
measured in a vertical plane only, and there are several devices 
for carrying it out. One is to suspend the lamp from the ceiling 
by means of cords and pulleys or a pulley block, so that it can 
be raised or lowered to different heights above the central axis of 
the photometer bench; the light is then reflected by a suitably 
placed plane mirror (making an angle of 45° to the bench) along 
the axis of the bench to the sight-box ; the centre of the mirror 
forming a right angle between the axis of the bench and the 
direction of the incident beam from the arc. The mirror is 
so arranged that the reflected rays always make an angle of 
45° with the axis. The absorption of light or coefficierit of 
reflection by the mirror at this angle is carefully measured and 
allowed for. Let this co-efflcient or percentage of the total light 
striking the mirror, which is reflected, = K. Then C.P. of reflected 

beam = x C.P. of beam from the arc, and this reflected 

beam is then measured against the standard. The distance of 
arc lamp from photometer sight-box tiien is reckoned as = 
distance between lamp and mirror + that between mirror and 
sight-box. This enables large C.P.s to be compared with 
comparatively small ones, which would otherwise necessitate a 
bench many yards long.. 

Standard of Light.— This should be as large as possible in 
magnitude, so as to keep the sight-box near the centre of the 
photometer bench, and thus minimize errors in its true position. 
In addition the colour of the light shouhl approximate to that of 
the arc. The standard which fulfils these conditions in a fairly 
satisfactory manner is one consisting of an over-run glow lamp of, 



ELECTRICAL ENGINEERING TESTING 


G3 


say, 32 C.P. at normal voltage. If this is over-run some 5-8% 
in voltage, it will emit a much whiter light and one that is 
more nearly the colour of the arc. This lamp must be carefully 
standardized against a known standard of light at two or three 
definite and accurately noted voltages above normal. Probably 5% 
over normal will give a C.P. = about 40 or 45, and 7% over normal 
about 50—60 C.P. At this abnormal voltage the bulb will 
blacken inside fairly soon, and hence the lamp should not only 
be kept on for the shortest time, but should frequently be 
re-standardized. 

Differences in colour between the two lights to be compared 
may to some extent be corrected by taking the readings of 
the slider carrying the sight-box when observing the balance of 
illumination of the latter through red and green glass in 
succession, the best kinds for the purpose being what are known 
commercially as “ signal-red ’’ and green ,and which should be so 
chosen that a bright sunlight viewed through the two together 
appears quite dark. The effect and object in using coloured glass 
is explained on p. 53. 

There are really two kinds of efficiency determinations 
recessary in connection with arc lamps which are automatically 
self-regulating, namely— 

(а) The “Commercial Efficiency” of the lamp as a whole 
reckoned in Watts per candle emitted, and which takes into 
account the total power in Watts absorbed by the lamp, ^. e. in 
the arc and regulating mechanism. 

(б) The “ ISTett Optical Efficiency,” reckoned also as above, 
but taking into account only the power given to the arc itself 
and neglecting that absorbed in the regulating mechanisms. 

The following tests are devised for the purpose of investigating 
these separately and comparing the results, and also of examining 
other very important points pertaining to arc lamps in general. 

(24) Measurement of the Commercial Effi¬ 
ciency of an Arc Lamp. 

Introduction. —In the present instance the photometer bench 
being of considerable length, presumably, the two sources of light 
are placed one at each end and in a line with the photometer 


G4 


ELECTRICAL ENGINEERING TESTING 


siglit-box, which will in future be termed the screen, for brevity 
sake. 

Apparatus. —Photometer bench BB fitted with a Bunsen 
grease-sjwt screen {G) (p. 692), placed inside a sight-box to pre¬ 
vent stray light, due to reflection from the walls of the room, 
falling on the screen, the walls and ceiling being as dull black 
as possible; standard known source of light (A^) consisting of 
an over-run 32 C.P. glow lamp, carefully standardized at a known 
voltage by a previous test. 



To 

bdttery 


A voltmeter preferably the same used in the calibration of 
the glow lamp, together with a rheostat r (p. 603) for reproducing 
the voltage of calibration. Arc lamp [L) to be tested, supported 
on a suitable stand, and placed in circuit with an ammeter A 
voltmeter rheostat B (p. 606), and switch S. A secondary 
battery should be available to feed both circuits in preference 
to a dynamo current, as the former gives a far more steady 
E.M.F. and better results than the latter. 

Observations.—(1) Adjust the arc lamp in its cradle or stand, 
so that the point of contact of the carbons is at the same height 
above the bench as the centre of the screen and standard light. 
Centre the two carbons very carefully. 

(2) hix the distance T) between arc and standard at some con- 

















































ELECTRICAL ENGINEERING TESTING 


G5 


venient amount for future calculations (say 600 cms.), and adjust 
the arc lamp so that the carbons are vertical. 

(3) Connect up as shown in Fig. 26, and adjust the pointers of 
A, Vi and Fa to zero if necessary. Yary ?• so as to give Ls the 
voltage as read off on V. 2 , at which it was standardized last, 
when it will then give a definite known C.P. 

(4) With R full in, close S, and vary R so that the arc just 
burns with the carbons in equilibrium. Now quickly adjust the 
position of the siglit-box, so that the grease spot appears equally 
illuminated all over each side. Note its scale reading (d) from 
the standard, the volts Fi, V-z and the amps. A. 

Note.—F> must be kept rigorously constant during this and 
the following observations. The scale reading d can be obtained 
most accurately by reading it when the grease spot is just per¬ 
ceptibly darker and lighter respectively than the surrounding 
paper. This should be done, using red and green glass in addi¬ 
tion to the naked eye, and the mean of all the readings recorded. 
The reading of Fi and A must be constant during the taking of 
the above readings. 

(5) Re-adjust R and repeat 4 for about ten different values 
of Fi, rising by about equal increments to 60 volts or so, V-z 
being constant. 

Each of the readings in 4 and 5 should consist of a group of 
3 or 4 observations with Fi and A constant, so that a mean may 
be taken which would allow for alteration in C.P. due to the arc 
travelling round the carbons. 

Tabulate your results as follows—• 

Name . . . Date . . . 

Arc lamp tested : No. . . . M .kor . . . Tj']ie . . . 

Type of carbons . . . Size of positive . . . Size of negative . . . 

Standard light: Type . . . Candle power =. . . i>-. . . cms. Ammeter Resist. = ... Ohms. 


Volts on 
Standard 
1^2- 

Amps, 
through 
arc lamp 

A. 

Volts 

across 

lamp 

Vl- 

Watts 
given to 
lamp 
AVi. 

Distance. 

C.P. of “ Arc ” 
Ka = 

(^-‘02 ^ 
d'i- 

Commercial 

Eflicienoy 

AVi, 

Ka 

Costper 
candle 
hour for 
power. 

d. 

D-d. 











(6) The voltmeter used for F^ especially if a hot wire, must be 

shunted, not directly to the lamp, but to the lamp and ammeter 

A combined. The volts lost in A can be calculated and sub- 

F 





















66 


ELEGTBIGAL ENGINEERING TESTING 


tiacted from that shown on Vi in order to get the true volts on 

% 

the lamp. 

The ammeter resistance is required for this correction and can 
be found approximately by the Wheatstone Bridge. 

(7) Plot the following curves on the same sheet of curve-paper 
between C.P. as ordinates in each case, and (i) Volts (ii) Amps. 
A, (iii) Watts AVi, (iv) Efficiency as abscissfe. 

(8) Compare the above cost per candle hour for power at Qd. 
per imit at normal voltage across the arc lamp with that of a 
glow lamp of equal C.P., taking 3 Watts per candle at normal 
voltage. The price of energy being the same. 

Inferences. —State clearly all you can infer from your experi¬ 
mental results. 


(25) Determination of the Nett Optical Effi¬ 
ciency of an Arc Lamp. 

Introduction. —This test is similar to the last, with the follow¬ 
ing exceptions— 

Place the voltmeter which should be of a sensitive high 
resistance type, across the arc instead of the lamp terminals as in 
the preceding test. This can be done by connecting it to two 
spring clips, which make good contact with the carbons about two 
inches from their ends next to the arc. The ammeter (A) must 
now be so arranged in the circuit that it measures the current 
through the arc without taking into account that passed by any 
shunt-coil which the lamp may ha 2 :)pen to have. 

Apparatus. —The same as for the commercial efficiency test, 
and in addition the two necessary spring clips. 

Observations. —Bepeat those of the foregoing test exactly as 
there indicated. 

Connect up as in Eig. 26, with the exceptions noted above as 
regards the position of the ammeter and voltmeter. 

Compare the efficiencies obtained from the two tests, and also 
the costs per candle hour for power at the same price. 


ELECTRICAL ENGINEERING TESTING 


67 


(26) Determination of the Distribution of 
Light from an Electric Arc. 

Introduction. —The distribution in the case of an arc liirht is 
obtained by measuring the C.P. at various angles to the horizon, 
in one single vertical plane containing the central axis of the 
photometer bar. 

To enable this to be done some arrangement, similar to the 
one briefly described under “ General remarks ” on arc light 
photometry, is necessary. 

The author, however, has devised a simple form of ^‘co'adle” 
in which to fix the lamps, and which is illustrated and described 
in the Appendix, p. 587. 

The difficulty arising in the use of such an arrangement is due 
to the fact that most arc lamps will not continue to self-regulate 
when placed in a slanting position. The author, however, finds 
that, with a suitable type of lamp, there is practically no difficulty 
from the above cause, until the cradle makes an angle of about 
50° with the horizontal, and for just the one or two readings 
after this it is easy to help the mechanism by hand in order to 
maintain the “ intake ” of electrical power by the lamp constant. 

Apparatus. —Precisely that mentioned for the test No. 24, p. 
63, on “ Commercial Efficiency,’’ except that the cradle is now 
required where just an ordinary stand would have done in that 
test. 

Observations. —(1) Repeat 1-3 of the above-cited test, seeing 
in addition that the carbons touch at a point which is in a line 
with the centre of the axle of the cradle, and that their axes 
coincide. 

(2) Set the cradle with its pointer at zero, when the carbons 
will be vertically over one another. "With R full in, close S, and 
vary R so that the lamp takes its normal voltage and current 
and burns quite steadily, then quickly balance (by moving the 
screen), in the manner set forth in observation 4 of the test 
cited; repeat this three or four times for the same values of A 
and Fj, and record the mean in the table. 

(3) Repeat 2 above, for the same values of A and F^, for every 
10° through which the cradle is turned, up to 70° or 80°, when 


ELECTRICAL ENGINEERING TESTING 


r,8 


the axis of the carbons will be nearly parallel with the photo¬ 
meter bench, and tabulate as in the table for test 24 cited, sub¬ 
stituting the heading “ Angle between carbons and horizon for 
the cost, etc., in the last column of the table. 



To 

battery 


(4) Repeat 2-3 for a considerably lower current A than the 
normal, but one at which the arc burns properly. 

(5) Plot the Polar Diagram or curves of distribution of light 
from the arc at various angles for each current used in the 
manner described below. 

Inferences. —State clearly all you can infer from your experi¬ 
mental results, and point out their bearing on the lighting of 
streets and large areas by means of arc lamps. 

Determine from your results the mean spherical efficiency 
which = (;| horizontal efficiency -f f max. efficiency). 

Note. —This may vary from 0’050 to 0-200, depending on the 
diameter and quality of the carbons, and is the ratio of the normal 
power in Watts, as given to the lamp in observation (2) above, to 
the mean spherical C.P. resulting. 

Plot of Polar Diagram and Distribution of Light from Arc. 

Let D be the junction of the -}- and — carbons, i. e. centre of 


arc. 















































ELECTRICAL ENGINEERING TESTING 


CO 


Let BB represent the maximum O.P. obtained for some posi¬ 
tion of the arc, then with D as centre and DB as radius, draw 
the semi-circle ABG. Divide this into eighteen equal parts, each 
of which will therefore = 10° of arc, and draw radii to the points 
of intersection so formed. Now set oif to the same scale as 
DB, the various C.P.s measured, along the respective radii from 
D, representing the angles in which they were measured. Then 
the curve DGFH drawn through these points is the polar dia¬ 
gram of C.P.s from the arc. 



Fig. 23. 


Q 


Y 

R 


The distribution corresponding to this polar diagram is 
obtained as follows— 

Draw PBS through B parallel and equal to ADC. 

Through each of the points of division on the semi-circle ABG 
draw lines parallel to DB, and therefore perpendicular to PBS. 
From PS on these set off lengths proportional to the respective 
C.P.s at the corresponding angles. Thus, for instance, 

DF maximum C.P., and so on for the rest. Now complete 





























70 


ELECTRICAL ENGINEERING TESTING 


tlie rectangle PQYRS and draw the second curve PYS. Then 

we have for the arc lamp— 

Area of curve PYS 

Mean spherical C.P. = 


X max. C.P. 


V 


„ rectangle PQRS 
= (J horizontal C.P. + J max. C.P.) approx. 
The curve PYS shows the manner in which the illumination 
of streets falls off with direct current arc lamps at different 
distances from the lamp for a given height above the ground. 


(27) Other Tests on Arc Lamps. 

Determination of the Effect of Carbons of Different 
Diameters and Quality on the Spherical C.P. and Spherical 
Efficiency. 

Notes. —In this test care must be taken to vary only one thing 
at a time, as for instance— 

{a) Vary the diameters only for exactly the same quality of 
carbon, all other conditions being constant. 

(h) Vary the quality only for exactly the same diameter of 
carbon, all other conditions being constant, as for instance the 
amount of power supplied to the arc. The spherical C.P. and 
efficiency is then measured in each of the cases a and b in the 
manner just described. 

The results should be tabulated in a convenient manner. If 
possible a curve should be drawn between each separate pair of 
variables, and, lastly, inferences deduced from the experimental 
results. 


(28) Determination of the Relation between 
Voltage and Current respectively, and the 
loss in grammes per hour of Positive and 
Negative Carbons. 

• Notes. —In this test, as was also mentioned in the last, only 
one thing must be varied at one and the same time. Thus— 

(a) For the same voltage, measure the loss in grammes of the 



ELECTRICAL ENGINEERING TESTING 


71 


same or exactly similar +and — carbon occurring in the 
same interval of time with different currents. 

{h) For the same current, measure the loss in grammes of the 
same or exactly similar +and —carbon occurring in the 
same time w’ith different voltages. 

The results should be tabulated in a convenient manner, and if 
possible the following curves drawn— 

Two between volts and amps, respectively on the abscissae and 
losses in grammes per hour of + carbon as ordinates. 

Two betw’een volts and amps, as before, with losses in grammes 
per hour of - carbon as ordinates. 

All on the same sheet of curve paper. 

Carefully deduce the inferences obtainable from the results of 
the tests. 


(29) Relation between Voltage and Length 
of Arc (with Constant Current through it). 

Introduction. —This test, like the next one, is important, as 
it indicates why certain types of lamps can be run at higher 
voltages than others, while in conjunction with the results of a 
corresponding test for obtaining the Polar Diagram of the lamp, 
the effect of the length of arc on the distribution of light over 
the lower hemisphere is clearly indicated. The reader should 
peruse the remarks under “ introduction ” in the next test which 
apply to the present one also. 

Apparatus. —Precisely that for test No. 30. 

Observations.—(1) Connect up as in Fig. 30, and set A and V 
to zero if necessary. 

(2) With (^R)fidl in, close S and “strike” the arc by bringing 
the carbons together for an instant, and then quickly separating 
them, R being reduced to keep the arc burning. 

(3) By varying R, obtain a series of arc lengths between about 
y' and the maximum possible by applying different voltages 
across it, the current being kept as constant as possible all the 
time at the most convenient value to be found by trial. Then 
after rapidly moving L to obtain the sharpest image / on G oi 
each arc length, quickly measure /, and note x, y, A and V. 


72 


ELECTRICAL ENGINEERING TESTING 


Uote.—Time should be allowed for the carbons to burn to 

shape, and for the arc 
to become steady before 
readings are taken. 

Tabulate your results 
exactly as in the last 
test, and plot curves 
having values of V and 
R as ordinates with 
length of arc (d) as 
abscissae. 

Find the constant (a) 
in the equation— 

V = Vq + 

to the working part xy 
Pjq 29 . curve. Fig. 29. 

Inferences. — What 

can you deduce from the results of your test 1 



(30) Relation between the Current through 
an Electric Arc and the Voltage across 
it (for a constant Length of Arc). 

Introduction. —The present test has an important bearing 
on the supply of electrical energy to “open,” “enclosed,” and 
“flame” arc lamps in view of the length of arc normally 
employed in these three distinctive types being different in 
practice. The voltage across the arc can be obtained by a high 
resistance voltmeter connected to two spring clips placed on the 
carbons as close to their tips as is safe without risk of fusion. 

The voltage thus measured will be that necessary for over¬ 
coming the apparent resistance of the arc made up of the back 
E.M.F. of the arc + the “ohmic drop” in the arc due to its 
ohmic resistance. 

If the voltmeter is connected to the lamp terminals it will 
measure the above-named apparent resistances -|- the additional 
“ ohmic drop ” between carbon tips and terminals. 





ELECTRICAL ENGINEERING TESTING 


73 


The length of arc may be found in one of two ways : (1) by 
throwing an image of the arc on to a screen at a known distance 
away, by means of a double convex lens, when from the length 
of image and the distances of the lens from arc and screen the 
length of arc itself is at once obtainable; (2) by placing a gauge 
of known length, about equal to that of the arc in front of the 
latter, and measuring the shadow of the gauge on the screen, 
when from the length of shadow, gauge and the distances, the 
length of arc is obtained. 

Apparatus.— Hand-feed arc lamp R with terminals TT; 
double convex lens Z, mounted on sliding base; ground or milk 
glass screen G; ammeter^; voltmeter Vj variable rheostat 7i; 
switch S and source of supply E. 



Observations. —(1) Connect up as in Fig. 30, and adjust the 
pointers of A and V to zero if necessary. 

(2) With [R) fidl in, close S and “strike” the arc by bringing 
the carbons together for an instant, and then quickly separating 
them, R being reduced to keep the arc burning, which must be 
carefully watched. 

(3) Adjust the arc to a convenient length, say 1" to J", and 
move L until the clearest image / is obtained on G, then quickly 
measure the length of I on G, and note the readings of V and A 
and the distances x and y. 

(4) With the length of I constant, vary R so as to obtain 
a series of values of V and A between 0 and, say, 25 amps.— 
X and y being constant, and the arc adjusted to keep I constant. 

(5) Repeat 3 and 4 for constant lengths of arc of about 
and J", and tabulate your results as follows— 













74 


ELECTRICAL ENGINEERING TESTING 


Distances. 

Lengths of 

Volts 

V. 

Amps. 

A. 

Apparent 

Resistances 

7? = — ohms. 
A 

X . 

y- 

Image (7). 

Arc (d) 

= - X /. 

y 









(6) Plot to the same axes, curves having values A as abscisste 
with both V and R as ordinates. 

Inferences. —What can you deduce from the results of the 
test 1 


(31) Examination of Alternating Current Arcs. 

General Remarks. —The alternating current arc possesses 
many characteristic and interesting features which are absent in 
the case of the continuous current arc. Thus for instance 
the two carbons consume away at approximately equal rates. 
The colour of the rays is quite different, being much more 
purple than in the direct current arc. Again, more energy is 
needed for the same volume of light emitted, and the arc gives 
out a rhythmic hum if it is burning properly. 

In addition the true power W given to the lamp may be 
considerably less than the apparent power A F, i. e. the product of 

the alternate current ammeter and voltmeter readings. Thus 

W 

the power factor which = may be very low and even down to 

0*50 in an alternating current arc lamp. 

The following additional investigations should be carried out 
on this type of lamp, namely— 

The effect on the C.P. of variations of {a) voltage, (6) current, 
(c) frequency, {d) quality of carbon. 

The effect on the angle of phase difference or power factor 

of (e) quality of carbon, (/) cored and uncored carbons, 
{g) hissing of the arc. 

The relative amounts of power absorbed by the arc itself 
and by the regulating mechanism should be investigated. 

Many of the above tests can only be employed on hand- 
regulated lamps. 



















ELWTRIGAL ENGINEERING TESTING 


75 


(32) Measurement of the Internal Resistance 

of Secondary Cells. 

Introduction. —The following method is the best for measuring 
the working value of the internal resistance of a storage cell or 
battery of such. Owing to the very low resistance met with 
usually in this kind of cell the ordinary methods are practically 
inapplicable, and in the present case the cell is being tested more 
or less under working conditions. 

If a battery is being tested the total internal resistance can be 
obtained at once, and if the cells are all of the same size, make, 
and type, the resistance of each cell can be deduced, probably 
with considerable accuracy, by dividing the total resistance so 
obtained by the number of cells and thus obtaining the average 
resistance per cell. It should be remembered that the internal 
resistance of any cell is not a fixed and invariable quantity but 
depends on several things, thus, for instance, on the density of 
the sulphuric acid solution which is continually changing accord¬ 
ing to the amount of discharge, or charge of the cell. It is 
interesting to note in this connection that the resistance of a 
solution of dilute sulphuric acid is least at a specific gravity of 
about 1220 and increases from this in either direction, i.e. for 
a rise or fall in density. Again, the internal resistance will 
depend on the condition the plates are in, and will be greater 
if they are “ sul 2 )kated ’’ than if in good condition. 

Apparatus. —The cell or cells (B) to be tested; voltmeter V of 
sufficiently large resistance, and having a long open scale, enabling 
small differences to be read accurately; ammeter A capable 
of reading up to the maximum current to be taken from the cell; 
key TT; switch S; carbon rheostat R (p. 597). 

Observations. —(1) Connect up as in Fig. 31, and adjust the 
pointers of V and A to zero, levelling the instruments if 
necessary. 

(2) With S open, close K and note the reading E on the 
voltmeter. This is therefore the E.M.F. of the cell in volts, 
since only an extremely small current is flowing. 

(3) Close both K and S and adjust R so as to obtain about 


76 


ELECTRICAL ENGINEERING TESTING 


y^^th of the maximum current 
taneously the readings on A and 



Fig. 81. 


output from B. Note simul- 
F, wliich latter now gives the 
terminal P.D. (F) in volts. 

(4) Repeat 2 and 3 for 
about ten different currents 
rising by about equal incre¬ 
ments to the maximum. 

(5) Calculate the work¬ 
ing value of the internal 
resistance h of the cell or 
battery from the relation 

E — V 

h = —-— ohms, 

Jl 

and tabulate your results as 
follows— 


Name . . . Date . . . 

Cell tested : Make . . . Type . . No. of Plates = . . . Size of Plates = . . 

Distance between Plates = . . . Approx. Sp. Gr. of Solution = . . . 


E.M.F. 

E Volts. 

P.D. 

V Volts. 

Current 

A Amps. 

Internal 
Resistance 
b Ohms. 

Internal 
Resistance 
per Cell for a 
Battery. 

Mean 

Internal 

Resistance. 








(6) Plot 2 curves having values of F and (6) as ordinates and 
A as abscisste. Show that the tangent of the angle of slope 
(from the horizontal) of the F and A curve = the internal 
resistance {h). 


(33) Measurement of the Efficiency and 
Storage Capacity of Secondary Cells. 

Introduction.—Secondary cells may be divided into two main 
divisions, namely—the “ Faure ” or pasted type, and the “ Plante ” 
or non-pasted type. The chemical changes occurring in either 
class, during charge and discharge, are precisely alike, but the 
reader is referred to ordinary text-books of Electrical Engineer¬ 
ing—for instance. Electrical Engineering in Theory and Practice, 
by the author—for such changes which hardly come under the 
scope of the present work. 

The secondary or storage cell has taken up so prominent a 





























ELECTRICAL ENGINEERING TESTING 


77 


position at the present day in both electric lighting and electric 
traction that the method of measuring the efficiency and storage 
capacity of any type of cell, or perhaps more particularly the 
relative behaviour of different types under the same conditions, 
is a matter now of paramount importance to every electrical 
engineer. A good deal may be said with regard to the precise 
mode of testing such cells, and in this connection much depends 
on the duty which they have to perform in actual practice. 
Any laboratory test of such cells will be worthless almost, from 
a practical point of view, unless it is carried out under conditions 
as nearly as possible alike to those the cell will work under in 
its everyday use. Thus, for instance, take a battery employed 
for merely lighting purposes, say at a central electricity supply 
station. It is never resting idle and never merely giving either 
its full load discharge or any other constant output, for the load 
which it has to take varies with the hour, day, and season of 
the year, from often next to nothing, to full load and sometimes 
a considerable percentage overload for short periods. Thus it 
will be seen that in this instance any test to be of value must 
be carried out as nearly as possible under these conditions, and 
for months continuously, too, instead of, perhaps, only for two or 
three weeks always at full load and with, say, a night’s rest in 
between each such discharge. 

Again, in the case of electric traction work, the above remarks 
do not all apply, for instance, usually a battery used in this kind 
of work in sub-stations is relieved of discharge between midnight 
and about 7 a.m. in the morning, during which period it is charged. 
When used for portable work, as in autocars and tramcars, it is 
subject to both rapid and wide fluctuations of output and often to 
excessive jolting. Hence the test on a cell required for this kind of 
work should be a very stringent one, automatic jolting gear being 
provided to operate on the cell while being discharged, while this 
latter must often be abnormal. Practically the Faure or pasted 
type of cell is the only one available for self-contained autocar 
traction, as weight forbids the use of the Plante type. As one 
instance of a traction type of pasted cell which will stand periods 
of excessive discharge and the wash of the solution against the 
plates and yet have a long life, the Headland secondary cell may 
be instanced, and tests extending over years amply justify this. 


78 


ELEOTBIGAL ENGINEERING TESTING 


Tlie efficiency of any secondary cell or battery can be reckoned 
in one of two ways, namely, the— 

Quantity efficiency, or Ampere-hour efficiency 

Ampere-hours given out 
” Ampere-hours put in 

Energy efficiency, or Watt-hour efficiency 

Watt-hours given out 
Watt-hours put in 

Each of these will depend to a certain extent on the relative 
periods of charge, rest, and discharge, and also on the current 
density or rate of discharge reckoned say in amperes per unit of 
area of positive plate. The greater this is the less will be the 
quantity efficiency, and also the energy efficiency, though the 
latter not to the same extent as the former. 

It may here be remarked that the quantity efficiency may be 
as high as 94% when the current density is low and the cell used 
under favourable conditions, whereas the energy efficiency cannot 
exceed 80% from the fact that the average normal voltage of a 
cell on discharge is 2'0 volts approximate and the average voltage 
needed to charge being 2'5 about. These two efficiencies in 
practice may be taken more nearly as about 75% and 65% 
respectively. 

The CAPACITY of any secondary cell may be expressed in one 
of two ways, namely, either as the ampere-hours or as the Watt- 
hours which it is capable of giving as a useful discharge. The 
term commercial capacity might be given to the number denoting 
the ampere-hours or the Watt-hours per lb. of plate (taking 
both -f-'® and together) or per lb. of cell complete, including 
acid, etc. 

At the present day, owing to there being so many forms and 
methods of building, the latter mode of reckoning the capacity 
is the only one available when comparing diffierent types of cells. 

A secondary cell may be charged either (1) at constant P.D. 
or (2) at constant current. In the first case a fairly heavy rush 
of current takes place at starting, and the method would be 
unsuitable for use on some types of pasted cells from the risk 
of the plates buckling. The second method is the one nearly 
always employed in practice and is the one which will here be 
considered. 





ELECTRICAL ENGINEERING TESTING 


79 


The cell should not he discharged normally below 1 *80 volts on 
closed circuit, since it will then become practically useless for light¬ 
ing circuits, and there is also the danger of the plates ^^sulphating ” 
rapidly below this limit. For the latter reason it should not be 
allowed to rest in this discharged condition. 

Apparatus. —Cell B to be tested ; sensitive voltmeter (F) with 


£ 



open scale ; ammeter (^1); switch S -^; carbon rheostat R (p. 597); 
two-way switch S ; source of charging E.M.F. [E) ; hydrometer 
and weighing arrangements if the latter should be required. 

Observations.—(1) Assuming that the cell to be tested is not 
already set up, but is still as received from the makers. First 
weigh each complete set of plates, “ Positives ” and also Nega¬ 
tives,” separately after dusting them. Also weigh the containing 
vessel, and the dilute sulphuric acid solution (of the specific 
gravity authorized for that particular cell), which is suflScient to 
cover the plates and be about one inch above their tops. Measure 
the size and thickness of the plates. 

(2) Set up the cell properly, connecting up as indicated in 
Fig. 31, and adjust the pointers of V and A to zero if necessary. 

(3) More as a matter of interest than otherwise, carefully note 
the sp. gr. of the acid solution before and immediately after 
putting it into the cell by means of the hydrometer, and then note 
the readings of this latter and the time frequently, while the 
sp. gr. is rapidly altering and until it becomes constant. 

Note. —In all cases exercise great care in keeping the hydro¬ 
meter away from the sides of the vessel and plates; if this is not 
done it will give totally erroneous readings due to adhesion. 


















80 


ELECTRICAL ENGINEERING TESTING 


(4) If the sp. gr. of the acid solution is constant note it, then 
with R at its maximum and S on contact a, note the time on 
closing aS'j and quickly adjust the current on A to the “ normal ” 
for this cell by means of R. 

(5) Keep this current constant in strength until the acid 
becomes milky in appearance throughout—commonly known as 
boiling and due to bubbles of gas liberated from the plates. Note 
the readings of the hydrometer and voltmeter and the time 
frequently while they are varying rather rapidly, but less often 
as they vary more slowly, and, lastly, open when the cell is 
completely charged. 

N.B.—Probably this first charge will last from at least 15 to 
something like 30 hours before the cell thoroughly comes up to 
the “boil,” and in no case should it be stopped in the first 12 
hours except for a minute or so. Beyond keeping the current 
constant from beginning to end the other readings during the 
middle stages of charge need probably be only taken every 1 or 2 
hours about. 

Tabulate your results as follows—• 

CHARGE. 

Name . . . Date . . . 

Name of Cell . . . Type . . . Normal rated capacity . . . Amp.-hours . . 
Weightof plates: Positive= ,.. lbs.: Negative= ,,. lbs.: Vessel= ... lbs.: Acid= . . . lbs. 
Thickness of plates: Positive = . . . Negative = . . . 

Total surface of Positive = . . . sq. ft. : Plate volume = . . . Acid volume = . . . Ratio = . . . 


Number of 
Charge. 

Amperes 

A. 

Terminal 
P.D. (E) Volts. 

Time 
in Hours. 

Sp. gr. of 
Acid. 

Input 

Amp.-hours. 

In]iut 

Watt-hours. 


• 







(6) Take note of the period of rest (if any) which the cell has 
had since the last charge, note the open circuit P.D. at its 
terminals and the sp. gr. Then put S to (b) and close aS'j at a 
noted instant of time, quickly adjusting A to the normal dis¬ 
charge value for the cell, which must be kept constant by R. 
Note the P.D. on V and the sp. gr., and the time frequently while 
they are varying somewhat rapidly, but less often as they vary 
more slowly. 

Open every half-hour, say for the shortest time necessary to 
just take the “ open circuit ” volts, noting the time at each. 

(7) Continue the discharge until the terminal voltage falls to 
1'80, then open and tabulate your results as follows—■ 
















P^LECTUIGAL EEtGINEEElNG TESTING 


81 


DISCHARGE. 


Number 

of 

Discharge. 

Time 

in 

Houis. 

Sp. gr. 
of 

Acid. 

Amps. 

A. 

Terminal Volts. 

Internal 

Resistance 

b. 

Output Capacity. 

Efficiency. 

Closed 

circuit 

V. 

Open 

circuit 

E. 

Amp.- 

hours. 

Watt- 

hours. 

Amp.- 

hour. 

Watt- 

hour. 













(8) Repeat tests 4-7 until the charge and discharge curves 
practically coincide, indicating that the cell has attained a good 
normal working state, and take note of the length of rests 
between charge and discharge. 

(9) Take a discharge, as per 6 and 7, for 50% undei'- and also 
“ ove?’’’-normal rate. 

(10) At the conclusion of all the tests carefully observe 
whether any appreciable “ buckling ” or disintegration of the 
plates has occurred. 

(11) Calculate the capacity of the cell in both amp.-hours and 
Watt-hours per lb. of total plates and per lb. of cell complete 
with acid. Also calculate the current density used per sq. ft. of 
-f plate, reckoning both sides of each. 

(12) Plot the following curves, all like ones being on one 
sheet—- 

{a) Internal resistance in ohms as ordinates and times in 
hours during discharge as abscissae. 

(5) Time in hours as abscissie, with voltage and sp. gr. as 
ordinates in each case for both charge and discharge. 

(c) Current density as abscissse and amp.-hours output as 
ordinates. 

((Z) Current in amps, as abscissae and the quantity and energy 
elBciency as ordinates. 


(34) Measurement of Resistance by the 
‘‘Post Office’' Pattern of the Wheat¬ 
stone Bridge. 

Introduction. —It is assumed that the first principles of a 
Wheatstone Bridge (W.B.) have already been studied from an 
ordinary text-book. The Post Office (P.O.) pattern. Fig. 33 II., is 
merely a specially-arranged and compact form of W.B. placed in 



























82 


ELEOTRTGAL ENGINEERING TESTING 



2 can be made any- 
from 1 to 11,110 ohms. 


N 


a suitable box for portable purposes. If the principle and action 
of a W.B. is understood at all, and the stamping opposite the 
various terminals in the P.O. form observed, it ought to be im¬ 
possible to couple up incorrectly. Each of the “ proportional 
arms” 7'^ and consists of three resistance coils of 10, 100, and 

T T 

1000 ohms each respectively, hence the ratio — or — can be made 

a very simple number. QRST 
(Fig. 33 II.) is the “adjustable 
arm” rg, and it consists of 16 
dilterent coils and one infinity 
plug either at Q, R, or N. The 
value of 7 
thing 

Opposite two of the terminals 
and 7') is marked (Galva¬ 
nometer Line) and (Line Copper 
or Earth) respectively. This is 
because the P.O. form is primar¬ 
ily intended for measuring the 
metallic and insulation resist¬ 
ance of telegraph lines, and 
hence in the first case that line 
would be joined to IN and T, 
and in the second case only one 
end to IN, the other being free 
and insulated, T then being put 
to earth. As therefore we are measuring metallic resistance 
(rj) it is put between IN and 7\ The terminals to which the 
battery R must be connected are equally obvious. The white 
dotted lines on the top show where the under contacts of the 
keys A\ and iq are joined to, inside the box. In any form 
of W.B. variation of the battery E.M.F. or its resistance or 
that of the galvanometer (G) has no effect on the accuracy of 
the measurement. The sensitiveness of the test, though princi¬ 
pally depending on that of G, can be increased within limits 
by using a larger E.M.F. and making r^, r 2 , rg and as nearly 
equal as possible. 7’Ae haUe7'y keij must alwaijs he p 7 'essed 
hpfo7e the goilvunometei' hey to allow the currents in the 



Fig. 33. 
















ELECTRICAL ENGINEERING TESTING 


88 


arms to become steady before pressing A\. The battery key 
should be pressed for no longer in order to prevent the coils 
being heated by the current and their resistance thereby altered. 
It should also he hroken last to avoid the risk of damaging G by 
inductive kicks when measuring inductive resistances. In 
inserting plugs press in lightly and give about ^ of a turn to 
insure good electrical contact. Reverse this operation when 
removing them. The ends of all connecting wires should bo 
scraped clean. 

Apparatus. —P.O. Bridge; sensitive galvanometer G (p. 571); 
2 or 3 Leclanche cells B. 

Observations. —(1) Connect up as indicated in Fig. 33 II., and 
adjust the galvanometer needle to zero. 

(2) Note once for all the direction in which G deflects when 
(?’ 2 ) Fig. 33 II. is too large to give balance (done by taking out 
“ Inf ” in 7’2 with, say, r^ = r^ = 10). 

Note.—Until balance is nearly obtained, only tap for a 
fraction of a second. 

(3) IMake r^ — r^^ 10 and balance the bridge by altering ?’2 so 

as to get no deflection on pressing and then If it is 

impossible to get exact balance, note the steady deflection when 
7*2 is just too large and too small, and calculate the correct 
intermediate resistance to give balance, by proportion. Thus 
if d^ — steady deflection of the galvanometer to one side of zero 
for the adjustable arm = 7ij, and 0^2 = that to the other side of zero 
for the adjustable arm = 7i’2» then if is greater than we 
have (7i\ — Ttg) ohms corresponding to a deflection of {d^-\-d,^ 
scale divisions, 

and .*. (^2 corresponds to (7?^ - ^ 2 ) 1—^ ohms. 

«! + «2 

Hence the resistance of the arm, which would give just no 
deflection (the required condition) 

= ;?2+(i?i-7?2) olims = )V 

(4) In order to obtain the true resistance (7q) of the unknown 
which is being measured, without the process of interpolation 
mentioned in the latter part of 3 above, the value of r^ or r^ may 

T 

be varied. Thus instead of — being = or 1 as in 3 above, we 

’’3 




84 


ELECTRICAL ENGINEERING TESTING 


might have ^ depending on the 

^’3 

value of the unknown r^. In many cases this will be equivalent 
to having decimals of an ohm in the adjustable arm (rg). Hence 


T 

increase or decrease the ratio of — and adjust so that on press- 

^’3 

ing 7^2 and then K-^ no deflection whatever occurs on the galvano¬ 
meter. Then note the values 7 * 2 , ^’3 and 

N.B.—If the unknown resistance is greater than 11,110 

ohms, then will be greater than but if (?q) is less than 

11,110 ohms, then may be either = , or less thanrg. Tabulate 
as follows—■ 


Resistance 

tested. 

Proportional Arms. 

Adjustable 
Ai’in rg. 

Unknown Resistance 

ri = ti X ra- 
’’3 

Mean r\. 

ra. r4. 







Note. —The limits of the P.O. Bridge are x 1) = 0 01 ohm 
and X 11,110) = 1,111,000 ohms, but measurements become 

less accurate as they approach these limits. 


(35) Measurement of the Armature Resist¬ 
ance of Dynamos and Motors, and of 
the Copper Resistance of Transformers 
and Electric Light Cables. (Potential 
Difference Method.) 

Introduction. —The Wheatstone Bridge is inapplicable for 
measuring very low resistances, and even if such were just 
within its range, the measurement would not be accurate owing 
to errors introduced by the variable contact resistances in the 
circuit. The following method, which depends directly on the 
definition of resistance, can be used to accurately measure very 
low resistances, such as are met with in large electric light 
cables, the armatures of dynamos and motors, and the low tension 
coils of transformers. 

The P.D. at the terminals of each resistance can be measured 
relatively by a sensitive galvanometer, whose resistance is large 

















ELECTRICAL ENGINEERING TESTING 


85 


compared with that between the two points to which it is applied. 
Under these conditions its insertion will not lower the P.D. to 
be measured. If it is a reflecting instrument the scale deflections 
will be proportional to the P.D. 

The most suitable instrument for a workshop test, which as a 
rule does not admit of the use of a delicate galvanometer, is a low 
reading voltmeter, having fairly large resistance, and reading to 
about 1 or 1 '5 volts for a full scale deflection. Such an instru¬ 
ment, although not nearly as sensitive to small differences of 
potential as the galvanometer, has the advantage usually of 
being more portable, and also less easily affected by magnetic 
fields in the vicinity. 

Apparatus. —Known standard low resistance R of about 0*01 
ohm (Fig. 273); low resistance r 
to be tested; Pohbs commu¬ 
tator U(p. 584); secondary cell 
B; rheostat Rh{p. 597); fairly 
high resistance galvanometer 
(p. 569) or low reading volt¬ 
meter G, preferably of the 
moving coil type; reversing 
key K (p. 585); switch S. 

Note.—The ends II and E of 
the low resistance {r) to be 
tested will of course be the 
terminals of the transformer 
coil, the ends of the cable or 
the brushes of the machine, the field coils being disconnected 
temporarily. The length of lead between D and II in the Fig. 
is immaterial. 

Observations.—(1) Connect up as indicated in Fig. 34, and 
adjust the galvanometer or voltmeter needle to zero. Clean the 
collecting arrangement at the part where the brushes press with 
fine emery cloth, assuming, for example, we are dealing with a 
dynamo or motor. To prevent the armature rotating, see that 
the Jield circuit switch is open, and that the brushes press on 
opposite ends of a diameter in the case of a diiect curient 
commutator. 

(2) With Rh full in, close S, and adjust the current to give 
about quarter-scale deflection with the largest resistance of the 



Fig. 34. 










86 


ELEGTRIGAL ENGINEERING TESTING 


two, for then the deflection with the other is bound to be on the 
scale; then note the galvanometer deflection on each side of zero 
by turning K, when G is across each resistance in turn. 

N.B.—The resistance Rh should be sufficiently high to prevent 
the current strength altering during any one pair of observations, 
and to prevent this current being strong enough to sensibly 
warm the resistances. The more sensitive the galvanometer the 
smaller this will be. After taking deflections with the second 
resistance, it is advisable to retake those with the fir*t in case 
the current has altered. If they are not the same, take the mean 
of those on the respective sides of zero. For very accurate woi'k 
a reversing key should be used with E to eliminate any thermo¬ 
current effects. 

(3) Repeat 2 for half, three-quarter, and full scale deflections, 
and calculate the unknown resistance r from the formula— 

Tabulate as follows— - 


Name . . . Date . . . 

Low Resistance tested . . . Standard low Resistance = . . . Ohms 


Deflection across R. 

Deflection across r. 

Ratio, 

dr 

da 

Unknown, 

r ohms = /i'kl 
da 

Rigid. 

Left. 

Mean 

da 

Right. 

Left. 

Mean 

df 










Inferences. —Prove the formula given in 3, and state any 
assumptions made in deducing it. What sources of error is tho 
method liable to 1 How can they be minimized 1 

(36) Measurement of Low Resistances by 
Voltmeter and Ammeter Method. 

Introduction. —The following method, applicable to the 
measurement of the low resistances met with in the armatures 
of dynamos, motors, transformer coils and electric cables, is one 
of the simplest and a direct application of Ohm’s law. It is not 
usually susceptible of the accuracy obtainable by the last 
method (Test No. 35) and depends on the accuracy of tho 
ammeter and voltmeter used, and on that of observation. 





















ELECTRICAL ENGINEERING TESTING 


87 


Apparatus. —Low resistance (?•) to be tested; accurate 
ammeter (d) and low reading voltmeter {V), both preferably of 
the moving coil type ; switch S ; variable current rheostat (/i), 
the form of which will depend on the current supply [E) 
available. If E comprises two or three large secondary cells, 
then li may be a carbon rheostat (p. 597), but if E should be a 
100 volt supply, then R may be a bank of lamps (p. 598). 

Observations. —(1) If, as is indicated, an armature resistance 
is to be measured, connect up as indicated in Fig. 35. Adjust 



the pointers of A and V to zero, and see that the field circuit 
of the machine is kept open throughout the whole test by keeping 
the field switch open or otherwise. 

(2) With V connected to the terminals TT of the machine, as 
actually shown, and v/ith {R) full in, close S and take simul¬ 
taneous ascending and descending readings on V and A for some 
five or six currents on A, differing in strength by about equal 
amounts between 0 and full-load armature current by suitably 
varying R—the armature being at rest all the time. 

Note.—This measurement will give the Static “brush-contact” 
resistance -f- resistances of armature and both brush leads BT. 

(3) Repeat (2) with the armature rotating {hij hand) while 
taking readings. 

(4) Repeat (2) with the armature at rest, but with the ends 
of the voltmeter wires disconnected from 'TT and carefully 
inserted under the brushes BB, so as to press against the proper 
commutator segments, the straight ends of the wires so inserted 
being parallel to the length of segment. Tabulate all your 
results as follow^s— 


Name . . . Date . . . 

Nature and Rating of Low Resistance tested . . . 


V connected to 

Actual 
Resistance 
being tested 

Amps A 

Volts V 

y 

Eesistance (?’) = - 
A 

Mean 

Resistance 























88 


ELECTRICAL ENGINEERING TESTING 


(5) Plot, on the scime axes, curves having values of V as 
ordinates, and (yl) as abscissae for tests 2, 3 and 4. 

Inferences.—What can be deduced from the curves and 
values of {r) obtained 1 

(37) Measurement of the Armature Resist¬ 
ance of Machines and other Low Resist¬ 
ances by simple Potentiometer Method. 

Introduction.—Since by Ohm’s Law V = I.R., where V = the 
P.D. across the ends of a resistance 7^ carrying a current /, it 
follows that when the same current I flows through two 
resistances, the P.D. {V) across each is oc to that resistance. 
The previous test (No. 35) was based on this fact, but since 
actual deflections (a to the P.D.s) had to be compared, the 
accuracy depended to some extent on the current-deflection law 
of the instrument used, and on the instrument having a high 
resistance relatively to those measured. The present test, based 
on the Clark-Poggendorff method of comparing two E.M.F.s, 
and unlike the deflection method No. 35, is a nvdl or zero 
method or one in which no deflection is the condition to be 
obtained. Hence the law of the instrument is immaterial, and 
an increase in its sensibility increases the accuracy of the test. 
Since also in this method the E.M.F.s to be compared are in 
turn placed in series with the instrument, the contact resistances 
of the connection to these E.M.F.s, as also the resistance of the 
connections, are immaterial; hence the greater accuracy with 
such a null method. Test No. 39 employs precisely the same 
principle as, but is a greater elaboration of, and a little more 
accurate than, the present method, in which we shall use a single 
or multiple metre bridge having a stretched undamaged wire 
of high resistance material, of uniform cross sectioned cirea and 
size, and which will not sag due to heating by the current 
from one secondary cell connected direct to its extremities. 
Thus the E.M.F.s to be compared can be balanced against the 
uniform fall of potential along the wire due to a constant 
current flowing through it, and the ratio of the lengths so 
balanced will be that of the E.M.F.s across them. 


ELECTRICAL ENGINEERING TESTING 


89 


w 


Apparatus. —Armature A to be tested; known standard lo 
resistance A ; switch S ; variable rheostat r ; secondary cells 
R and E ; Fold’s commutator or change over key C ; metre 
bridge PQ with sliding contact key; sensitive galvanometer G. 

Observations. —(1) Connect up as shown in Fig. 36 and 
adjust G to about zero. Ensure the 
connections being such that when C is 
turned so as to include the fall of 
potential of either R or A in the circuit 
of Gy each P.D. opposes that along PQ 
due to E. 

(2) With r full in, close A, and 
adjust r so that with C turned to the 
larger of the two resistances, a posi¬ 
tion, say. La cms. from P, is obtained 
at which there is no deflection on G. 

(3) Now quickly turn C to R finding 

some position Lr cms. from P at which G does not deflect; next, 
again verify whether the point La still gives balance. If 
slightly different take the mean of the new position and that in 
obs. 2. 

(4) Obtain several pairs of positions such as La and Ijr by 
altering (r) and tabulate as follows— 





Name . . . Date . . , 

Nature of uuknown Resistance . . . 

Value of known Resistance 7; = Ohms. Galv. No. . . . 


Wires xy con¬ 
nect* d to 
which j oints 
of A. 

Unknown 
Resistance 
measured, 
Hot or Cold. 

Distance of Slider from P. 

Unknown 
Resistance 
L\ ^ 

^R 

Mean value 
of Rj^ ohms. 

L.i 

Lr 








Inferences.—On what does the accuracy of the test depend, 
and how can it be made more sensitive ? 
































90 


ELECTRICAL ENGINEERING TESTING 


(33) Measurement of Metallic or Conductor 
Resistance by the Silvertown Portable 
Testing Set. 

Introduction. —The method used in measuring the resistance 
of the conductor of the circuit under examination is that of 
Wheatstone’s Bridge. 

Fig. 37 below shows only those parts of the instrument which 
are employed in this test, and omits the parts and their connec¬ 
tions which relate only to insulation testing. The parts employed 
are the following— 

1. The adjustable resistance. This it will be seen consists of 
two sets of 9 coils, each connected to circular plug commutators 
or dials. One set of coils has nine resistances of ten ohms each, 
making ninety ohms in all, the other has nine resistances of one 
ohm each, making nine ohms in all. If the hole marked with 
any number, say 5, is plugged in the ten-ohm dial, a resistance 
of fifty ohms is inserted between the connecting leads entering 
and leading away from the dial; and a similar rule applies to 
the one-ohm dial. Hence if the hole 6 be plugged in the tens 
dial and the hole 8 be plugged in the units dial, a total resistance 
is inserted in the two in series of 68 ohms. The lowest resist¬ 
ance that can be obtained is given when both the 0 holes are 
plugged, when the coil resistance inserted is zero. The highest 
resistance is obtained by plugging the two 9 holes when the total 
resistance is 99 ohms. If no plug is inserted in one or both 
dials, the circuit is broken and the resistance is infinity. 

2. The second part of the apparatus is the double set of pro¬ 
portional resistances, consisting of two coils of 10 ohms each, two 
of 100 ohms and two of 1000 ohms. Of these only one on each 
side of the centre is to be unplugged for any given test, and a 
rule is given later on for selecting the resistances to be employed 
to obtain the greatest possible sensitiveness; that is to say, for 
selecting those coils which will give the largest deflection on the 
galvanometer, when the resistance plugged in the dials varies by 
a given error from that of the circuit under test. 

3. The third part is the galvanometer. Its two terminals are 
connected to the two ends of the Wheatstone Bridge by depressing 


ELECTRICAL ENGINEERING TESTING 


91 


the contact key. It will be noticed that the shunt coils, with 
their plug commutator, are omitted from the diagram. This 
is done because they are not essential to the test, though they 
may be conveniently used when the balance of the bridge is not 



Fio. 37. 

yet approximately correct, and very large deflections are being 
obtained. 

4. The battery may consist of three Leclanch4 cells, having an 
electro-motive force of about 5 volts. One pole of the battery 
is connected in the usual way to the middle of the W^heatstone 

































































92 


ELECTRICAL ENGINEERING TESTING 


Bridge, and the othei’ to the point where the end of the adjustable 
dial coils is connected to one of the terminals, to which the con¬ 
ductor under test is attached. The connections are made by 
inserting the plugs at the end of the battery leads, in the two 
holes marked bridge, and immediately this is done the current 
is established in the coils; the galvanometer circuit is of course 
not completed till the key is depressed. 

5. The ends of the conductor to be tested are to be secured 
under the two terminals marked bridge terminals, and in 
measuring low resistances care must be taken that they are very 
securely attached. This may be done for very large or stranded 
conductors, either by soldering to their ends thin brass plates 
with holes in them of a suitable size to go under the heads of the 
terminals, or the connection may be made by means of finer wires 
soldered to the end of the main conductor. The resistance of 
these must be independently ascertained and subtracted from the 
gross result. 

Providing an idea is first obtained as to the magnitude of the 
resistance to be measured, the following table will be found 
helpful in expediting any test with the “ set.” 

Table III. 


For Resist¬ 
ances tested 
between 

Values of Proportional 
Arms. 

No. of Signifi¬ 
cant Figures 
in Result. 

Battery 

Power. 

Remarks. 

Left-hand 

Coil. 

Right-liand 

Coil. 

1 and 10 

100 

10 

2 

Ordinary 


10 and 100 

100 

100 



An extra signifl- 

100 and 1000 

100 

1000 

n 

>> 

cant figure can be 

1000 and 10,000 

10 

1000 

i> 

Increased 

obtained, calculated 

O'l and 1 

100 

10 

n 

>> 

by proportion from 

O'l and 1 

1000 

10 

)) 

) > 

the deflections. 

O'Ol and O’l 

1000 

10 

1 




A third figure can always be found in measuring resistances 
between one ohm and 1000 ohms, by observing the deflections of 
the galvanometer needles on both sides of the zero for different 
adjustments of the dial resistances near the balancing point. 

For example, we will suppose that the 10-ohm coil in the right- 
hand side of the bridge, and the 100-ohm coil on the left-hand 
side are unplugged, and that when 45 ohms are plugged in the 
dials, and the key depressed, a throw of three divisions of the 















ELECTRICAL ENGINEERING TESTING 


93 


galvanometer needle is observed to the right; and when 46 
ohms are plugged we get a throw of two divisions to the left on 
the galvanometer scale. It is clear that the resistance to be 
measured lies between 4*5 and 4 6, and is nearer to 4’6 than 
4*5, as two is less than three; that is, the resistance is 4*56 
ohms. As a further example, suppose 100 ohms to be un¬ 
plugged on each side of the bridge, and 82 ohms to be plugged 
in the dials; on depressing the key, no deflection of the needle 
is observed. On plugging 81 ohms in the dials, a throw of six 
divisions to the right is obtained, and on plugging 83 ohms we 
get the same deflection to the left. We are then amply justified 
in putting the third figure in the result as 0, and the resistance 
to be measured is 82*0 ohms. 

Except in testing at the extreme range of the instrument, i. e. 
quantities less than one ohm or greater than 1000 ohms, the 
galvanometer will be found amply sensitive, and it is better to 
place the south end of the controlling magnet uppermost, there¬ 
by reducing the time of the oscillations of the galvanometer 
needle. 

The battery should be in circuit as short a time as possible 
to avoid running down the cells, and it is well to take out one 
of the battery lead plugs when any alterations are being made 
in the plug commutators, only replacing it just before pressing 
the galvanometer key. 

Observations.—(1) Connect up as indicated in Fig. 37, using 
the “ set ” precisely as there indicated. The box should be 
placed on a table, or some other approximately level surface in 
front of the operator, he facing the magnetic east, and the con¬ 
trolling magnet being in a vertical position. The pointer of 
the galvanometer will then be found to be swinging near its 
zero, and may be brought exactly to it by slightly turning the 
controlling magnet. 

(2) Find roughly the resistance to be measured by unplugging 
ten in each of the proportional arms, and then adjusting the 
dial resistance so as to give a minimum deflection on pressing 
the key, the dial readings will roughly be the value of the 
unknown. 

(3) Now proceed to balance according to the foregoing table 
and remarks, and tabulate as follows— 


94 


ELECT RIGAL ENGINEERING TESTING 


Name . . . Date . . . 

Resistance tested . . . Temperature . . . 

Length of Conductor = . . . Gauge = . . . 


Proportional Arms. 

Adjust¬ 

able 

dials 

’'3- 

Deflections to the 

Calculated 
Resistance 
to balance 

r. 

Corrected 

dials 

r -s ± r. 

Unknown 

Resistance 

=-V3 + A 
r2 

Left 

rg. 

Right 

n- 

Right 

di. 

with 

Left 

d2. 

with 


1 









N.B.—The resistaDce (r), needed to exactly balance, is 
calculated thus—Assuming r^' to be greater than 7 * 3 ", then 
(? 3 '- 9 * 3 ") ohms corresponds to a deflection of (c/j + cZg) scale 
divisions, and corresponds to a resistance of 

ohms^r, 

, *. the correct dial resistance requisite to give no deflection 
= r 3 + r = 7^3 + « - ohms. 


(39) Comparison of Resistances. (Crompton 
Potentiometer Method.) 

Introduction. —This method is a valuable one for comparing 
two or more resistances of almost any value, within reasonable 
limits, and consequently of determining the actual resistance in 
ohms ef one of them, the other being an accurately known 
standard, such as one of the forms described on p. 605, which 
are some of the accessories of the potentiometer. The method, 
which is very simple and susceptible of great accuracy, is more 
particularly applicable to low resistances such as short lengths 
of electric light cables and the armatures of dynamos, etc., 
rather than multiples of the ohm, and it can be worked in such 
a way that the unknown resistance can be read olf by inspection 
directly in ohms. Thus it will be seen that the present measure¬ 
ment is a practical development of that known as “ Measurement 
of Low Kesistance by the Fall of Potential Method,” given on 
p. 84, and is a direct application of Ohm’s Law. The principle 
of it consists in comparing the relative falls of potential down 
the two resistances traversed by the same current through the 






























ELECTRICAL ENGINEERING TESTING 


95 


medium of the potentiometer, employing the principle of the 
Clark-Poggendorff method for comparing two or more E.M.F.s. 
The Crompton potentiometer is a specially arranged form of 
comparing instrument, and the operator should, prior to com¬ 
mencing the test, make himself acquainted with the instrument, 
a detailed description of which is given on p. 510, together with 
the method of using it. The accuracy of the results is principally 
dependent on the standard known resistance, and the value of 
the largest current sent through this and the unknown must be 
such that the fall of potential down either does not exceed 1'5 
volts, and that neither is warmed up by that current sufficiently 
to alter their resistances. 

The observations may be taken in one of two ways— 

(a) Suppose the potentiometer has been-“set” by the Clark 
cell in the usual way (p. 514) for E.M.F. or current measure¬ 
ments, and that the standard resistance -^s = 0*01 ohm. Then 
to avoid disturbing the “ setting,” balance each fall of potential 
down the two resistances, against that down the potentiometer, 
and compare the two P.D.s from the relation— 

Fs • Fjb = Rs ' Rjt' 

Example. —Let the standard balance with E on stud 1 and 0 
at 95 on the scale, the P.D. across ^gis 10004-95 = T095 volts. 
If now the unknown balances with E on stud 2 and C at 190 
on the scale, the P.D, across it = 2000 -f 190 = *2190 volts, 

V Rs ‘2190 

Rji = : ' iQ 95 ^ 0 01 = 0-02 ohms. 

(/5) Suppose the potentiometer has not been “ set ” by the 
Clark cell. Balance up on the standard resistance instead. 
Thus put E to stud 1 and C at 0 on the scale, and alter G and 
as described on p. 514, so as to “balance the potentiometer’’ 
for no deflection. Now with G and fixed, balance with the 
unknown resistance, which position of balance will give the 
resistance Rjt in ohms directly. In the present instance this 
would be E on 2, or jS_k = 2 x *01 = 0’02. 

Apparatus. —Crompton potentiometer P (Fig. 208); secondary 
battery B capable of easily giving the largest current suitable 
for sending through the low resistances ; switch S ; one secondary 
cell {h) for the “ working cell ” of the potentiometer; accurately 
known low resistance R (p. 605); unknown low resistance r to be 




DC ELECTRICAL ENGINEERING TESTING 

tested; standard Clark cell E carbon rheostat [rh) (p. 597); 
sensitive aperiodic D’Arsonval or moving coil galvanometer (g) 
(p.569). 

Observations. —(1) As a precaution first place the levers Ox 
G and E (Fig. 208) on studs 14, and that of H on stud 1, then 
connect up as in Fig. 38, in which only the row of terminals 
on the potentiometer PF is shown symbolically. 

(2) Adjust the galvanometer (^) and current indicator A to zero 
(roughly), levelling them if necessary, A being merely for the 



purpose of indicating roughly about what current flows in r and 
R. See that the standard resistance R chosen is of such a value 
as to be about the same as the estimated value of the unknown r. 

Note. —The maximum current then to be used must not produce 
a fall of potential in either R ov r exceeding 1*5 volts. 

(3) “ Set the potentiometer ’’ as indicated in either a, or 
balance on the standard resistance as in /5 above, the contact 
lever II referred to above being on studs IV or III respectively, 
as the case may be, thus inserting E or the P.D. across R in the 
circuit of (p), and taking care that it opposes the P.D. due to (6). 
Now close aS, and adjust (rh) so as to obtain some convenient 
current on A. N.B. This last-named operation is done before 
balancing P by method (I. 
























ELECTRICAL ENGINEERING TESTING 


97 


(4) With the positions of the resistances G and (Fig. 207, 
p. 510) as found in 3, unaltered, turn H to studs II or III, 
according to the “ setting ” employed, so as to throw into circuit 
with {g) the P.D. across one or other of these terminals. Then 
adjust E and the slider G to obtain no deflection on (g) when the 
latter is pressed, and note the reading of each. 

Note. —If it is impossible to ‘‘ balance ” owing to the spot of 
light being deflected always to one side, the P.D. down the 
resistance is assisting instead of opposing (as it should be) the 
fall due to (&) in the stretched wire; the wires from resistance 
to potentiometer are then to be interchanged. Lastly, turn 11 
again to the setting used in obs. 3 to see if balance is still 
obtained. If it is not, reset P and repeat. 

(5) Pepeat 4 with II on the studs leading to the other 
resistance, if ‘‘setting a” is the one being used. 

(6) Repeat 3-5, obtaining some six or eight distinct sets of 
readings by suitably altering the current through 11 and r, and 
tabulate your results as follows— 


Clark Cell: No. . . . Temperature = . . . ® C. E.M.F. assumed = . . . Volts, 
rotentiometer setting : ^on . . . Cat . . . Standard known resistance .. Ohms. 


Ammeter 
reading for 
reference only 

A 

Potentiometer reading. 

Unknown 

resistance 

pr 

Mean 
r ohms. 

R in circuit. 

r in circuit. 

Stud 
of A. 

Slider 

C. 

P.D. 

Stud 
of A. 

Slider 

C. 

P.D. 

Vr- 

Vm 











Inferences. —On what does the accuracy of the test depend ? 


(40) Measurement of Low Resistance by 
the Nalder Low Resistance Measurer. 

This method has the advantage of being a null or zero one, 
and entails the use of a specially arranged piece of apparatus or 
“measurer,” together with a secondary cell capable of giving a 
current of 5 amps and a variable rheostat to adjust this current. 

The general arrangement (Fig. 214) and method of use is 
given on p. 521, and will not be repeated here. 

n 





















98 


ELECTRICAL ENGINEERING TESTING 


Insulation Resistance. 

Introduction. —Probably we shall not be straying very far 
from the truth when we remark that Insulation Resistance is one 
of the most important matters that an electrical engineer has to 
deal with. In fact, so obvious is this that the statement hardly 
needs qualifying; suffice it to say that a breakdown of the insu¬ 
lation resistance—whether of street main, appliance fed from it, 
or of an ordinary electrical installation supplied off it, will either 
cause a temporary or prolonged stoppage of the supply owing to 
the mere “ blowing ’’ of a protecting fuse or cut-out, or, if the 
circuit is over-fused, in the burning out of part of the circuit and 
possibly the firing of premises in which the breakdown occurs. 

It is therefore of the utmost importance to be able to test the 
insulation resistance of a length of cable, main, or circuit, either 
when no current is flowing through it or when the supply is in 
actual progress and the main or circuit “ alive,” as it is usually 
termed. 

A number of different methods have been devised and are in 
general use for measuring the insulation resistance of both ^^dead ” 
and cables and systems, and in the following pages devoted 

to this question some of the principal and common ones in use 
will be considered. Before, however, proceeding with actual 
methods of measurement it may be profitable to make some 
general remarks. 

Electrical cables and wires are in the first place tested for their 
insulating qualities by the manufacturer prior to being sent out 
to the purchaser, but the latter should test them also himself, 
both before and after laying, to make sure that no faults have 
developed, and of course periodically during use. Such a mode 
of procedure is of the utmost importance if efficient working and 
maintenance is to be obtained, for it is quite possible for a cable 
to be accidentally damaged during laying and a subsequent fault 
to develop at this point, due to the strain of working conditions, 
which will finally break down the cable. 


ELECTRICAL ENGINEERING TESTING 


99 


(41) Measurement of the Insulation Resist¬ 
ance of Electric Light Cables by the 
Direct Deflection Method. 

Introduction.— The ordinary Post Office form of Wheatstone 
Bridge will measure resistances up to ITll megohms, though 
even at this maximum limit the measurements are not very 
accurate, owing to the resistances of the arms of the bridge being 
so widely difPerent from one another ^ consequently it is unsuit¬ 
able for measuring insulation resistance, which almost invariably 
amounts to much higher values, often of the order of hundreds of 
megohms. The present method of direct deflections^ which is also 
termed the “ simple substitution ” method^ is the most accurate 
in such cases and often the most convenient one to employ. The 
principle of it consists in comparing the deflection of a galvano¬ 
meter needle caused by a given E.M.F. through a known standard 
high resistance in series with the galvanometer, with the deflec¬ 
tion produced by the same E.M.F. working through the insulation 
of the cable to be tested substituted for the standard resistance. 

Preparation of Cable for Test.—This must be carefully done 
and is of the utmost importance if the true insulation resistance 
of the dielectric is to be found, as the difference between the 
results obtained with properly and improperly prepared ends is 
very great. The method of doing this should be as follows— > 

{a) For vulcanized india-rubber cables, the braiding, tapes, or 
other covering should be removed for at least six inches from 
each end down to the surface of rubber covering, care being 
taken in doing this not to cut or otherwise injure the rubber 
covering still left. 

(h) Wash this rubber surface with naphtha and scrape with a 
clean knife to remove any foreign material still left on the surface, 
and in this way so get a clean, fresh surface. 

(c) Taper the rubber with a clean sharp knife for about 1" to 
2" from the end, and then carefully dry the whole of the prepared 
end over a spirit flame without burning the rubber. 

(d) Paint or coat the whole of the prepared end with three or 
four coatings, one after another, of clean paraffin wax, melted to 
a temperature not exceeding that of boiling water. This can be 

> ) 

> 

) > 


> ) > 


100 ELECTRIGAL ENGINEERING TESTING 

done by placing tlie can of wax inside one of boiling w ater, 
whereas, if the wax is melted over a flame, it may be allowed to 
burn, and its insulating properties partially destroyed. As each 
coating of wax will have set before another can be got on, the 
whole cable end will be eventually sealed by a considerable thick¬ 
ness of the wax, which being much less hygroscopic than rubber, 
will not allow moisture to accumulate and so impair the prepared 
end. In lieu of wax the prepared end may be lapped with pure 
clean warm rubber tape well stretched, but this is not so good as 
the wax finishing. 



A better and much more expeditious way of eliminating errors 
due to surface leakage over the ends of a cable which is being 
tested for insulation resistance, than that just described of care¬ 
fully tapering the ends and coating them with paraffin wax, is to 
employ an ingenious device known as Price’s guard-wire. This, 
when properly applied, gives complete protection from errors due 
to end leakage in the direct deflection method, where the cable 



























































































































































ELEGTBIGAL ENGINEERING TESTING 


101 


ends are close to the galvanometer, and, consequently, the connect¬ 
ing wire between cable core and galvanometer is air insulated. 

In the case of other methods, such as the loss of charge test, 
extra precautions are necessary to avoid errors (vide Fliil. Mag, 
vol. xlix. pp. 343-7, April 1900). 

If the cable ends are close to the galvanometer then Price’s 
guard-wire device in its simplest form is shown in Fig. 39 (/). T is 
a lead-lined tank of water in which the cable G to be tested, for 
insulation resistance, is immersed. The ends of G are prepared 
with a long clean taper {t) from the core W, so as to give a long 
clean surface of insulation exposed to leakage. A thin copper 
guard-wire igv) is wound two or three times round the tapered 
part rather nearer the outer braiding than the core W and con¬ 
nected as shown to the galvanometer G and high voltage battery 



R. If now the resistances of the taper surface (t) are large 
compared with that of G they will all be at the same potential, 
and we shall have no leakage, but if any leakage exists (w) will 
tend to keep up tlie potential of IF, the deflection of the galvano¬ 
meter G being now reduced in the ratio j where a and h 
represent the conductivities of G and t respectively. Consequently 
the correct result will be-x deflection. 

(h 

It will, however, be evident that in some cases the cable ends 






























102 


ELECTRICAL ENGINEERING TESTING 


cannot be brought up close enough to the galvanometer in order 
to have an air insulated wire connecting G and W. 

The simplest and best way of getting over this difficulty is that 
suggested by Prof. Ayrton and Mr. T. Mather and represented in 
Fig. 39 {II). The inner conductor {ii)oi a concentric wire {K) is 
used to connect TF and G, the outer {oo) connecting {w) with 
junction of G and B as before. Now if oo has a high insulation 
resistance compared with the internal resistance of the testing 
battery By complete protection is a:fforded against surface leakage, 
even though KK is lying on the ground. 

Apparatus. —Sensitive high resistance Thomson astatic re¬ 
flecting galvanometer G with its box of shunts ^ S ; known 
standard high resistance R ; unknown insulation resistance 
(r) of cable to be tested; well-insulated battery B of either 
Leclanch6 or secondary cells capable of giving an E.M.F. of from 
100 to 500 volts ; two-way highly insulated spring tapping key K 
(p. 586); suitable lead-lined water-tank TF. If the “lead” or 
cable to be tested is small enough, it may be run direct from the 
key K into the water-tank TF (clear of everything) and coiled 
up under water, the free end being carefully 'ke'pt dry and left 
standing upright, about 12" out of the water, as indicated at D. 

The tank should contain ordinary cold tap-water at a tempera¬ 
ture of about 70° F., and the cable to be tested should be allowed 
to soak in this for 24 hours before the test, with its end trained 
up in mid-air above the water some 12" or so. 

If the cable is too large to be taken up to Gy a short well-insulated 
G.P. covered wire must be tied on to it at G, and the joint insu- 
'lated as at B. In all insulation tests at least the working pres¬ 
sure which the cable is to be subject to should be used to test 
it with. 

The known standard high resistance may preferably consist of 
a metal megohm, but in lieu of this costly piece of apparatus, a 
carbon megohm, checked against a metal 100,000 ohm coil occa¬ 
sionally, will do quite well, and costs only a few pounds. It 
must, however, be borne in mind that such a resistance slowly 
alters its value with time and temperature, so that the temperature 
should be noted each time it is checked by the present method. 

Note. —To avoid damaging the galvanometer, which is a very 

1 Or the Ayrton and Mather Universal Shunt-box. 


ELECTEIGAL ENGINEERING TESTING 


103 


delicate one, the shunt-box provided with it must always be used 
in the way indicated below. 

Tests.—(1) With the lever switch or short circuit har of S down^ 
thus short circuiting the galvanometer terminals, and also with 
the shunt-plug in the hole, connect S up to G firsts and then 
the rest of the circuit as indicated in Fig. 40, and adjust the spot 
of light to zero by means of the controlling magnet. 

(2) Remove the short circuit in /S', and, with the shunt 
plugged up, gently tap K for a fraction of a second so as to complete 
circuit through the standard known resistance R ; if the deflection 
is inappreciable, release K to plug up the shunt, and again 
tap as before, and so on until a convenient steady deflection djj is 
obtained. Note this and the shunt in use at the time (if 
any). 

N.B.— The key K must always he released before altering the shuntS. 

(3) See that the short circuit lever of S is down so as to short 
circuit the galvanometer terminals, and that the -g-^ shunt is 
in. Now close K through the insulation resistance, and after 
about half-a-minute open the short circuit switch. 

Note. —If this method of proceedure is not followed a sudden 
ballistic rush of current may ensue through G, just at first, from 
the high battery E.M.F., into the cable, owing to this latter acting 
as a capacity, i. e. condenser, and thus damage the galvanometer. 
At the end of one minute note the steady deflection d^, and with 
the key K still closed, again at the end of every minute up to 
between five or ten, say. 

If without the wire connecting K and W there is leakage from 
battery and galvanometer which gives a deflection d^,. Then 
{dj. + dri) or (dy-dri) must be used instead of d^ simply, according 
as to whether d^i is opposite or in the same direction as dj. 
respectively. 

(4) Repeat 2 and 3 for about 3 or 4 pairs of deflections if 
possible in different parts of the scale, with R and r, by altering 
S, and calculate the insulation resistance (r) from the formula 
below, and tabulate as follows— 



104 


BLECmiGAL ENGINEERING TESTING 


Name . . . Date . . , 

CaWe.—Insulating materials . . . Resistance of Galvanometer G~. , . ohms @ . .. ®C. 
Size of copper core = . . . S. W.G. „ Standard iZs. , . „ @ • n 

Length immersed L — ... Miles. 

Time of immersion = ... Hours. Temperature = ... 


E.M.F. 

used. 

Standard Known 
Resistance. 

Unknown Insulation Resistance. 

In ohms 
H. 

Deflection 

Shunt. 

Time in 
min. from 
closing K. 

Deflection 

dr. 

Shunt 

Sr. 

r megohms. 

Megohms 
per mile 
r y. L. 











(5) Plot a curve between time of electrification in minutes as 
absciss£B and corresponding deflections as ordinates. 

K’ote. —If aS'= i (say), then 

Inferences. —Prove the formula mentioned in 4, e.: 


d 


u 


R 1 + 






+ G\=d 


1 + 




and state what assumptions are made in deducing it. 

Should it be found impossible to keep the deflection on the scale 
with the standard known resistance in circuit and the -g^-g-th shunt 
in, using the full battery E.M.F., then employ only a known 
fraction of this total E.M.F. (as measured by an electrostatic 
voltmeter) when taking a deflection dj^ with the standard, whence 


in the above formula we must use ~xdj^ instead of dj^^ simply 

where V = full E.M.F. and v that used to obtain dj^. In testing 
short lengths of highly insulated cable at the least an E.M.F. of 
300 or 400 volts should be used. 

Keferring to the ten 1-minute readings of deflection in ob¬ 
servation 3 above, the deflection will fall rapidly at first and 
then more slowly. This is not due to increase in the insulation 
resistance, as it might appear to be, but to dielectric absorption 
in the cable through this acting as a condenser. 

This particular test is a good one for developing a fault which 
would pass observation in a test of 1 minute’s electrification, in 
which case there would either be an irregular—or no—crawling 
of the deflection in the 10 minutes. 

Insulation resistance is usually specified in megohms per mile 
at 70 F. after 24 hours’ immersion and 1 minute’s electrification 
at some definite voltage. 
























ELECTRICAL ENGINEERING TESTING 


105 


It is necessary to record the temperature at the time of the 
test, as the insulation resistance decreases as temperature increases. 

To Conductor of Cable. 'To Earth or Sheathing, 



Connections for Testing insulation Resistance, 

Fig. 41. 

(42) Measurement of Insulation Resistance 
by the Silvertown Portable Testing Set. 

This is a measurement of the electrical resistance of the insulat¬ 
ing material of a cable to the passage of a current from the inside 
conductor through the insulation to the lead sheathing, wet yarn, 
armour, or other outside conducting surface, and the inverse of 
the insulation resistance is generally termed the leakage. 








































































106 


ELECTRICAL ENGINEERING TESTING 


This measurement is effected by a method known as that of 
direct deflections. It consists in passing a current from a battery 
through a galvanometer into a conductor of a cable whose farther 
end is free and disconnected, thence through the insulating 
material to the outside coating or earth, and so back along a 
temporary conductor to the other end of the battery, the deflection 
of the galvanometer needle produced by this current being noted. 
Replacing that part of the circuit which was formed by the insu¬ 
lating material of the cable by a standard resistance of known 
value, we obtain a new deflection of the galvanometer needle. 

The diagram Fig. 41 shows only those parts of the apparatus 
and their connections that are used in this measurement; those 
which relate only to the measurement of conductor resistances 
being omitted. 

The arrangement, it will be seen, is as follows:—One pole of 
the battery—the battery of Leclanch6 cells giving an E.M.F. 
of about 100—200 volts is normally employed—is connected by a 
conductor, ending in an ebonite-headed plug, to the lower of the 
two plug-holes marked insul''- Thence the current passes along a 
connecting wire to the block marked shunts, and thence through 
the galvanometer to the upper block on the other side. We may 
observe in passing that these two main blocks, one on each side, 
are practically the terminals of the galvanometer. If a shunt is 
plugged, -Jth, •^’^th, or yj^yth only of the current passes through 
the galvanometer, the remainder finding its way through the 
corresponding shunt coil. 

From the upper block on the left-hand side the current may 
take two paths, according as the hole marked insul"* or that 
marked 50,000 ohms is plugged ; if neither is plugged, the circuit 
is broken, and no current can pass. This plug forms consequently 
a convenient make and break key. If the hole marked insul”- is 
plugged, the current passes to the terminal marked insul”', so 
through the insulating covering of the cable to the outside 
sheathing or earth, back to the terminal marked earth and the 
plug-hole marked E, and then along the lead to the other pole of 
the battery. If, however, the hole marked 50,000 ohms be 
plugged, the current will pass through the coil of 50,000 ohms, 
then along a connecting wire to the plug-hole E, and so back to 
the battery. 


ELECTRICAL ENGINEERING TESTING 


107 


Tn beginning this test the conductor of the cable, or insulated 
wire, or a temporary lead attached to it, is connected to the 
terminal of the instrument marked insul^’, and another lead, con¬ 
nected to the outside sheathing of the cable, or the wet soil in 
which it lies, is attached to the terminal marked earth, care 
being taken that these leads are separated, and that no circuit 
exists between them except through the insulation of the cable. 

It will be observed that when all the holes in the straight 
commutator near the front of the box are plugged, the key 
on the left-hand side, which is used in the bridge test as a galva¬ 
nometer make and break key, becomes for the insulation test a 
short circuit key, and is useful for checking quickly the oscilla¬ 
tions of the needle. 

N.B.—Although the maximum voltage of the testing battery 
usually employed with this testing set is only 100 volts, the set 
can be used with a testing voltage of 200 volts, as required by 
the Board of Trade regulations. In this case, instead of using the 
multiplying power of 20 as described above, the multiplying 
power of 100 should be used in taking the constant—the deflection 
thus obtained will be the same as that which would be given by 
the unshunted galvanometer through a total resistance of five 
megohms, and the calculation of the resistance to be measured 
would then be made in exactly the same manner as described above, 
except that five megohms Avill be substituted for one megohm. 

In making this test the following points may be called 
attention to— 

(1) Too much care cannot be taken in preparing the ends of 
the cable. Since we are measuring a very small current of 
electricity passing from the conductor to the outside sheathing, 
through the insulated covering, it is clear that our results will be 
entirely misleading if any current be allowed to pass over a dirty 
surface at the ends where the conductor is exposed- These ends 
should be looked to before testing, and in the case of india-rubber 
or other firm material, the section of the insulator should be 
pared all over with a sharp and perfectly clean knife. For 
methods of preparing the ends see pp. 99—100. 

(2) Care should be taken not to short circuit the battery, which 
may easily occur in two ways. One is by allowing the two battery 
plugs to touch one another, when the other ends of the leads are 


108 


ELECTRICAL ENGINEERING TESTING 


attached to the battery terminals ; and another is by allowing the 
lead attached to the earth terminal to touch that attached to the 
insulation terminal. 

In both cases the battery of small cells will be for a time much 
overworked, and in the second the needle may become bent or 
demagnetized. 

(3) Another point that may be noticed is that in deducing the 
insulation resistance per statute mile from a test on any given 
length, the result obtained from a test on the latter is to be 
multiplied by the length of the piece in miles, and not divided 
by it. 

For example, if the insulation of a cable three miles long be 
15 megohms, the insulation per mile will be 15 x 3 or 45 meg¬ 
ohms ; or again, if the insulation of a piece of cable, whose 
length is 350 yards, be 7520 megohms, the insulation per statute 
mile will be megohms = 1495 megohms. 

If the galvanometer deflections are proportional to the currents 
producing them, and the E.M.F. employed is constant through¬ 
out the whole test, then we have current oc deflection cc 


or if Rj — insulation resistance tested and 

standard known resistance 


total resistance ^ 
dj — deflection through it, and if Eg 
and = deflection through it, then 
Rj: dg 


R 


s 


whence Rj — Rg megohms, 

dj dj 


where Rg = the standard resistance in megohms. 

If, however, the galvanometer is shunted with, say, a ^ shunt, 
for example (with the insulation), so that only I of the main 
current goes through it, then, since without the shunt the 
deflection would be five times as great, we have 

dg 


Ri=- 


5d, 


Rg megohms. 


Thus, for example, suppose that a given battery produces on 
the needle of a galvanometer placed in series with the insulation 
of a cable in the manner described, a deflection of 10*3 divisions, 
and that on substituting a resistance of one megohm for the 
insulation we get 42 divisions, we find that the insulation resist¬ 
ance is y^^^ = 4*1 megohms approximately. 

. Again, for example, suppose that the current from the battery 









ELECTRICAL ENGINEERING TESTING 


109 


when passed through a constant resistance of 50,000 ohms gave 
a deflection of 42 divisions on a galvanometer shunted to and 
that when passed through the cable insulation it gave 23 divisions 
with the galvanometer shunted to i, the insulation resistance 
would be iiiegohms =‘37 megohms approximately. 

In cable testing the battery employed should in all cases give 
an E.M.F. at least = the working voltage under which the cable 
works. The terminal marked Earth on the right of the galvano¬ 
meter must be connected to earth [i.e. nearest gas or water-pipe) 
or to the water of the tank if the cable is being tested in 
such. 

Tests. —(1) Connect up precisely as in Eig. 41, and adjust the 
galvanometer needle to zero. 

(2) Now take the “ constant ” of the galvanometer by plugging 
the 50,000 ohm hole and the shunt and note the steady 
deflection djs. This is the same deflection as that which would 
be obtained with the same E.M.F. through (50,000 x 20) = 
1,000,000 ohms, or 1 megohm for no shunt at all. 

(3) Plug up hole marked insul^. instead of that in 2, and 
adjust the shunts (if necessary at all) to obtain a steady deflection 
di , preferably as nearly = to d^ as possible. 

(4) Calculate the insulation resistance from one or other of the 
preceding formulae and note for reference merely the E.M.F. 
used in the test. 

N.B.—If the cable has been soaking in a water-tank note the 
time of immersion and the temperature of the water. Also the 
number of yards immersed. 


Insulation Resistance of Electric Light 
Street Mains and House Installations. 

Introduction. — Mains. —Seeing the extreme importance of 
maintaining continuously, and without any intermission of any 
kind, the supply of electrical energy from a central station when 
once commenced, it should be the endeavour of any engineer 
to obtain and lay the best possible class of cables in the most 
efficient, thorough, and lasting manner in his power. The item 
of mains in the supply of electrical energy is a very serious one 



110 


ELECTRICAL ENGINEERING TESTING 


at the best, and usually amounts to something like from ^ to | 
of the cost of the whole undertaking. 

Notwithstanding this, however, the best possible main only 
should be laid if the system is to be a lasting one, free from 
perpetual worry to the engineer, of cables breaking down and 
the consequent temporary discontinuity of the supply. The 
insulation, jointing, and laying should be the best it is possible 
to obtain, for even in localizing a fault the accuracy of the test 
will greatly depend on the goodness of the joints. 

There are roughly speaking three tests, which should be carried 
out on any new cable or main, namely— 

(a) The insulation and copper resistances of each cable drum as 
soon as it arrives from the manufacturers. This can only be 
done satisfactorily under a pressure of at least double that which 
the cable will work at in practice, and with the whole cable 
drum wholly immersed in water at about 70° F., the ends being 
carefully kept dry, trained out of the water and prepared in the 
manner described on p. 99. Reliable results cannot be obtained 
from a well-wetted drum, only from one wholly immersed for 24 
hours. 

The insulation resistance should be obtained by the “ direct 
deflection” method after one minute’s electrification(i. e. applica¬ 
tion of the battery), and again at the end of every succeeding 
minute for some ten minutes. 

The first reading will give, or should give, at least, the specified 
insulation resistance of the maker. 

The second and subsequent readings are extremely useful in 
showing the existence of undeveloped faults in the cable insula¬ 
tion, which would in the usual course of events pass the specifica¬ 
tion in the one-minute test unnoticed. Should the insulation be 
faulty, the galvanometer deflection will hardly fall at all after 
the first minute’s electrification, or may fall in irregular jumps. 
The resistance of the copper core should be taken and noted down, 
as well as the length of the cable on the drum. 

{b) The insulation and copper resistances during laying both 
before and after jointing in the following manner:—A careful 
test should be made on the first section of the line, one end of 
which we will assume is in the station when laid, but before any 
joint is made, and with both ends carefully prepared (see p. 99). 


ELECTRICAL ENGINEERING TESTING 


111 


If satisfactory, it will show that no damage has been done to it 
in the laying operation. 

The second section is then laid and carefully but temporarily 
is connected to the first section by a piece of lead. These two 
adjacent ends and the Jar end of section 2 are carefully prepared 
and the two sections tested. If the insulation resistance per 
mile^ is up to specification, section 2 is all right and can be 
now jointed to 1 and the test repeated. If not the same as 
before the first joint is defective and should be re-made. Now 
lay section 3 and again test as before, and so on; thus, finally, 
the whole line will have been tested section by section as the 
laying proceeded, and, lastly, as a whole. In this way a record of 
all the tests will be to hand at any future date, while any damage 
done in laying, or any badly-made joint, will be at once detected 
by fall in the insulation resistance per mile, and a rise in the 
copper resistance per mile. 

(c) Daily tests (while working or otherwise), the precise 
method of performing which will depend on whether the main is 
a high or low tension one. 

A fault occurring should at once be localized and remedied. If 
on a “ feeder it can easily be found, but if on a distributing 
main, sectioning off may be necessary to localize it. 

The apparatus requisite for these tests is the same as that 
enumerated on p. 101, and with which the testing-room of 
every station should be provided, in addition to other instru¬ 
ments. 

The minimum insulation resistance for low tension cables at 
100 volts is about 300 megohms per mile after 24 hours’ immer¬ 
sion and one minute’s electrification. In high tension cables at 
2000 volts it is about 4000 megohms per mile under similar 
conditions. 

Ordinary Installations. —Many of the preceding remarks 
apply here. For example, it is much more economical in the 
long run to wire a building with high-class insulated wires and 
leads as also with good fittings having fairly high insulation. 

^ The insulation resistance per mile = measured (total) resistance x the 
length in miles tested. This arises from the fact that the leakage current 
through two miles is twice that through one mile, assuming, of course, that no 
appreciable fault exists anywhere along the length of cable. 


112 


ELECTBICAL ENGINEERING TESTING 


The last-named condition to be aimed at is an important one 
when we consider that every lamp switched on brings one or 
more fittings, such as a lamp-holder, cut-out, switch, ceiling 
rose, etc., into active use, thereby adding so many additional 
parallel paths of surface leakage through which current can leak 
away to earth. This in other words means a diminution in the 
total insulation resistance of the whole installation, and which is 
considerably aggravated by damp weather, dust and dirt, etc. 
With respect to leakage of current, the rule given by the 
Institution of Electrical Engineers is that the total leakage 
should not exceed -s^^th part of the total working current. 
Numbers of different rules and regulations are given by the 
various Eire Insurance and Supply Companies. As an example 
of the latter we may cite the rules of the Edinburgh Corporation 
for installations tested at 115 volts wuth minimum insulation 
resistance. 


Table IV. 


For 12 lamps 5*0 megohms. 

,, 25 ,, 2-5 ,, 

j, 50 ,, 15 ,, 

„ 75 „ 1-25 „ 

„ 100 „ 1-0 


For 150 lamps 0'75 megohms. 


9 9 

200 

9 9 

0-5 

99 

99 

250 

99 

0-3 

99 

99 

300 

99 

0-2 

99 


The rules of the Leeds Corporation for 200 volt circuits are 
some 20% less than the above in the minimum insulation 
resistance for the same number of lamps in the respective cases. 

Ordinary insulation resistance tests for installations must be 
taken with —all fuses inf all lamps removed from their holders 
and all switches “ onf with at least the working pressure for 
which the installation is intended, but preferably double this. It 
is then advisable to make three tests as follows—■ 

{a) of the insulation resistance between -f leads and earth, 

Q>) )> >) 5> >> J) )5 )J 

(c) „ „ ,, ,, ,, -f^®and leads. 

The value in each case should not be less than that specified 
above or thereabouts for the particular number of lamps installed. 

Usually the insulation resistance for alternating current 
circuits has to be greater than for direct currents, and in some 
regulations these are as 1 : 2. 

The importance of testing at or even above the working 
pressure will be seen from the following figures by F. Uppen- 




ELECTRICAL ENGINEERING TESTING 


113 


born of Berlin, which show in a marked degree the way in 
which the so-called insulation resistance varies with different 
voltages. 


Table Y. 


Resistance between 

Terminals of slate cut-out. 

Two twisted cotton covered wires. 

Volts. Megohms. 

5 C8 

10 53 

13-6 45 

27-2 24 

Volts. Megohms. 

5 281 

10 188 

16-9 184 

27-2 121 


This drop in the reading of insulation resistance as the volts 
increase will generally he found to occur with, and is due to, 
moisture in or on the insulation under test. The increase of 
voltage may either break the insulation down, or the current, 
due to the voltage, may partially dry the moisture out and 
the reading gradually rise with the time of application. 

Further, in measuring insulation resistance, sudden variations 
will sometimes be observed, especially will such be noticeable in 
direct reading testing sets. This is invariably due to metallic 
or other conducting particles on the surface (or, very rarely, 
buried in the insulation) promoting surface leakage, and the 
rapid fluctuations are due to intermittent sparking between such 
particles. Thus a direct reading testing set discriminates 
between low insulation due to damp, that due to dusb, and that 
due to conducting particles, or whether the insulation is 
disintegrating under electric stress. 

(43) Insulation Resistance of Electric Light 
Installations, Cables and Machinery by 
Evershed’s Direct Reading Portable 
Testing Sets. 

Introduction. —While there are several portable insulation 
testing sets on the market of both the direct and indirect 
reading types, we shall here consider the time-honoured form due 
to Mr. Sydney Evershed, which indicates the instantaneous 
insulation under high pressure by the direct deflection of a 

pointer on a scale. Thus the tests can be safely entrusted to 

1 








114 


ELECTRICAL ENGINEERING TESTING 


anyone possessing practically no technical skill who can make a 
report, and so enable defective work to be discovered before 
cove7'ing in. 

The Evershed type is made by Messrs. Evershed and Vignoles, 
Ltd., in two forms, namely— 

(1) The MeggerInsulation Testing Set, which is the most 
modern development of the early form of Evershed ohmmeter 
and generator. 

(2) The Bridge-MeggerTesting Set, which combines the 
functions of the first-named “ Megger ” set with those of a 
Wheatstone Bridge. 

Both sets consist of an ohmmeter of the moving coil type 
combined in one box with a hand-driven generator for providing 
the necessary testing pressure and current. When the handle 
of the generator is not being turned, the pointer is entirely free 
and will rest anywhere on the scale. 

The internal construction and arrangement of the ohmmeter 
portion in both sets is the same, except for some minor additional 
details in the case of the Bridge-Megger set, which will be 
indicated later on. 

We will therefore consider the use, firstly, of type (1) above, 
namely— 

The “ Megger ” Insulation Testing Set. —The generator 
portion of this particular set may be either of the rariahle 
•pressure or constant pressure kind. Unless the electrostatic 
capacity of the work to be tested exceeds one microfarad or so, 
a variable pressure instrument is suitable, which is the case for 
testing wiring, switch gear, dynamos and motors, arc lamps, 
instruments and accessories. Megger insulation testing sets 
being ohmmeters, their readings are independent of the applied 
pressure. If, however, the insulation under test has a large 
electrostatic capacity, the reading may become unsteady, due to 
the capacity current, caused by the variable pressure flowing 
through the current coil only; but even on large capacity, once 
the circuit is charged, the capacity current ceases and the 
reading becomes perfectly steady. 

For work likely to have a capacity exceeding one microfarad 
or so, such as the wiring in metal conduits, lead-covered cable, 
underground mains and a modern system of house-wiring in 


ELECTRICAL ENGINEERING TESTING 


115 


metal conduit, which has often a considerable capacity, the 
constant-pressure type of set should always be used. 

The type of testing set now being considered is intended 
primarily for the measurement of insulation resistance, and is 
not available for metallic resistance tests. The low-range 
variable pressure sets are made in three ranges of 0-10, 0-20, 
and 0-100 megohms with 100, 250, and 500 volts respectively 
(at 100 revs, per min.), while the constant pressure low range 



To Line ’terminal. 

To Guard Terminal. 

To Earth Terminal. 


TANK CONTAINING 
CABLE 

UNDER WATER. 


Fig. 42. 


sets are made in the same three ranges together with a fourth 
for 0-200 megohms with 1000 volts. The high-range constant 
pressure sets are made in three ranges of 2-1000, 4-2000 and 
4-5000 megohms with 500, 1000 and 1000 volts respectively. 
These last-named “ Megger ” Insulation Testing Sets are provided 
with a guard wire terminal, and, as explained on page 100, any 
error in tests of high insulation due to leakage internally or 
across the surface of the insulation under test can be eliminated, 
and hence the readings of the set remain unaffected, by tightly 
wrapping a so-called Ioslvg guard wzre round the tapeied insulation 
between conductor and earth, and connecting it to the guard 
wire terminal as shown in Fig. 42, 


To Measure Insulation Resistance by the 
“Megger” Insulation Set. 

Observations._(1) For both low-range-varmS^e and -constant 

pressure sets, as well as for the high-range constant pressure 
set: place the instrument on a steady base, but not on the 
bedplate of, or very close to, a dynamo or motor. 

(2) Connect the terminal marked line to the insulated copper 









































116 


ELECTRICAL ENGINEERING TESTING 


core of the appliance under test, and that marked earth to a 
good earth such as a water-pipe or earthplate; or for testing 
between, say, two insulated wires, connect one wire core to each 
terminal. Then— 

(3) Turn the handle in a clockwise direction at a speed of at 
least 100 revs, per min., at which— 

In variable-pressure sets, the generator will be giving its rated 
or normal voltage which can be increased by increase of speed, 
and— 

In constant-pressure sets, the clutch is felt to be slipping (for at 
any speed above that necessary to give slipping, the voltage will 
be constant). Now read the insulation resistance as given by 
the deflection of the pointer on the scale. 

(4) High -range constant pressure sets (in addition to obs. 
1-3 above) must be levelled by means of the spirit-level seen 
through the hole in the dial, and the hidex must he adjusted to 
infinity before connections are made to any of the terminals, by 
rotating the handle above the clutch-slipping speed and turning 
the knob of the index adjuster one way or the other until the 
index stands exactly on the mark at infinity. 

Note. —In testing circuits of considerable electrostatic capacity 
it is essential to maintain full speed for at least a minute before 
taking the reading. Further, to eliminate errors due to surface 
leakage, a guard wire (see p. 100) must be used. 

To Measure Insulation Resistance by the 
‘‘Bridge-Megger” Testing Set. 

Observations. —(1) Connect up, as in Fig. 43, and turn the 
change-over switch to “Megger,” the instrument being on a 
steady base and not very close to a dynamo or motor. 

Note. —In all cases the line terminal must be connected to 
the insulated conductor of the circuit or appliance under test 
and the earth terminal to a good earth, such as a water-pipe or 
earthplate, or the equivalent. This with machines may be the 
framework, with conduit wiring should be the metal conduit 
itself, with lead-covered street main should be the lead sheathing- 
and for non-sheathed cable should be the water in the 
immersion tank, etc. 


117 


ELECTRICAL ENGINEERING TESTING 

(2) The handle is then rotated clockwise just above the speed 
at which the clutch is felt to slip. This occurs at about 100 
revs, per min., while at any higher speed the voltage is constant, 
and the insulation resistance is then read off by the deflection of 
the pointer on the scale. 

CHANGE 



(44) To Measure Conductor Resistance by 
the “Bridge-Megger” Testing Set. 

For use in this measurement, the adjustable standard known 
resistance box, supplied with this set, is required. Then to 
measure— 

Resistances under 100 ohms. —(1) Stand the instrument on a 
steady base and with all terminals free from any outside 
connections. Adjust the pointer to “ infinity ” on the scale by 
turning the knob of the index adjuster one way or the other 
until the pointer or index stands exactly on the infinity mark. 

(2) Connect up as in Fig. 44, set the change-over switch to 
“ Bridge,” the ratio switch to 10 or to 100, and all the 
resistance box- dials to zero. 

(3) Rotate the handle slowly clockwise with the right hand, 
when the pointer will float off the scale on the side marked 
“ increase R ” above the line marked simultaneously with the 



























118 


ELECTRICAL ENGINEERING TESTING 


left hand raising the value of R by turning the resistance box 
switches until the pointer exactly covers the line G. 

(4) Now rotate at full speed to give maximum voltage and 
hence sensibility, readjusting the box resistance li, if necessary, 
to keep the pointer on the line G. 

Then the resistance tested = the value of i? -r 10, or by 100, 
whichever is in use. 


EESISTANCB BOX 



EESISTANOl UNDER TEST 

Fig. 44. 


Resistances from 100 to 9999 ohms. —(5) Operate tests 1-4 
above, except that in (2) the ratio switch is now set to 1 
instead of to 10 or 100 as above. Then the resistance in the 
box required to balance the pointer exactly on the line G is now 
the value of that under test. 

Note. —When measuring field coils of dynamos and motors, 
or other metallic resistances of large self induction, the generator 
must be driven above the speed at which the clutch slips to 
ensure the current being constant in the arms of the bridge. 

Resistances from 10,000 to 999,900 ohms (by Bridge” 
method). —(6) Operate test (1) above. 

(7) Connect up as in Fig. 45, set the change-over switch tc 
“ Bridge,” and the ratio switch now to 10 or to 100. 

Note. —It will be observed that the connections of the 
unknown resistance and box to the" testing set in Fig. 45 are 
just the reverse to those of Fig. 44. • 

(8) Operate tests (3 and 4) above, remembering that the 
directions “increase i?” and “decrease R” are now also 




























ELECTRICAL ENGINEERING TESTING 


110 


reversed, and that the unknown resistance now = box reading to 
balance X 10 or 100, whichever ratio is in use. 


F.ESISTANCB UNDER TEST RATIO CHANGE-OVER 



Fig. 45. 


Resistances from 10,000 ohms and upwards (hy Megger'* 
method). —This method is more rapid, but less accurate than the 
last, and is operated exactly as for Test No. 43, p. 115^ the con¬ 
nections being as in Fig. 46. 


CHANGE- 
RATIO OVER 
SWITCH SWITCH 



RESISTANCE 
UNDER test 


Fig. 46. 


(45) Measurement of the Insulation Resist¬ 
ance of a Complete Electric Light Instal¬ 
lation and plant while working. 

Introduction. —The insulation resistance of any system of dis¬ 
tribution of electrical energy when not working can be determined 
by one of the preceding methods. These are, however, inap¬ 
plicable to systems actually running, and consequently “alive” 
at the full working pressure. It is most important to make 
frequent, if not daily, tests of the insulation resistance of any 
installation in order that a gradually developing fault, which 







































120 


ELEGTRIGAL ENGINEERING TESTING 


would cause the insulation resistance of the whole system to 
gradually fall, might be discovered in time and remedied before 
it perhaps burnt itself out and fired the premises. 

The following method is a simple and convenient one for 
making such a test on any system, whether that of a country 

house, having its own 



having 

generating plant or the 
main distributing network 
of a large town. The ar¬ 
rangement is shown in Fig. 
47, where +M and —31 
are the positive and nega¬ 
tive mains or wires of a 
two-wire system of direct 
current distribution. 

The contact studs 1 and 2, 
of a two-way key K (p. 586), 
are electrically connected 
(temporarily or otherwise) to 
any points a and b of these mains, which might be the + and 
-’bus. bars on the switch-board. The common terminal of K is 
connected through a Pohl’s commutator (p. 584) or other reversing 
key P to earth E {i. e. nearest gas- or water-pipe). By means 
of P, a voltmeter (F) connected to P has its terminals inter¬ 
changed between K and E on moving P over so as to reverse. 
Thus a current flowing either from K to E or E to K can be 
made to give a deflection in the same direction on V by 
manipulating P. If this latter is used it can be converted 
into a reversing key by cross-connecting the four mercurv 
cups, as shown by separate connectors indicated by the dotted 
lines; both K and P should have a good insulation resistance ; V 
should be a -}- and — instrument, preferably of the moving coil 
D’Arsonval type. It may eitner be a voltmeter or ammeter, but 
we shall assume the former in the present case. 

Observations.—(1) Connect up as shown and put K to stud 1, 
P being such as will allow F to deflect over the scale. Note the 
reading F^ on the voltmeter. 

(2) Put K to 2 and reverse at P so as to still make F read on 
the scale. Note the reading 










ELECTRICAL ENGINEERING TESTING 


121 


(3) Calculate the insulation resistance of the whole lighting 
system (including dynamos, battery, leads, cut-outs, lamps, etc., 
etc.) from the relation— 

where r„ = the resistance in ohms of the voltmeter, and V = the 
working pressure at the time across + M and — M. 

The insulation resistance of the + main is 

and of the — main is 

Jh - 

Since Fj and Fg are to opposite sides of zero, or on the same 
side by reversing at P, they must be added; ?•„ should not be too 
large, but its value depends on the insulation resistance tested. 
If this be 100 ohms or so, might be of the order of 1000 ohms. 
Since the value of R, and R 2 in ordinary small installations is 
usually high, z. e. considerably over 1000 ohms, a voltmeter may 
be used having a resistance (r„) of, say, 5000 to 10,000 ohms. 

Inferences. —Why is an electrostatic voltmeter unsuitable for 
use in the above test ] 


(46) Measurement of the Insulation Resist¬ 
ance of Complete Electrical Installations 
and Distributing systems while working. 

Introduction. —The following method is one of the most 
accurate for measuring the insulation resistance of a system 
taken as a whole, but it does not give any idea as to which main 
a fault may be developing. It is really an application of Mances’ 
method for determining the resistance of a conductor containing 
an E.M.F., and entails the use of preferably an ordinary Post Office 
pattern Wheatstone Bridge, with its detecting moving needle 
galvanometer G (p. 571) and an extra-resistance (r) for safety. 
Fig. 48 shows the sketch of connections for the test, where -j- d/ 
and — M are the mains the insulation resistance R of which it is 
desired to measure ; E represents earth {i. e. the nearest gas- or 





122 


ELEOTRIGAL ENGINEERING TESTING 


water-pipe) ; FF are protecting fuses. The P.O. Bridge is repre¬ 
sented symbolically by the zigzag lines A, G, Sj which indicate 
the rows of plugged resistance coils. 

Observations.—(1) Connect up as shown, joining the bridge 
terminal D to some point {a) on the distributing system to be 



tested; r may be a few ohms or that offered by half-a-dozen or 
more glow lamps in parallel. 

(2) With the proportional arms and arranged so that 
rj : 9*2 is as small as possible in order to obtain the adjustable 
arm lai’go, close and then bring back the galvanometer 
deflection, thus produced, to zero by means of the controlling 
magnet. 

(3) Then with closed alter 9*3 so that on opening and closing 
no motion of the galvanometer is observed. Then the insu- 

T 

lation resistance of the system as a whole is i? = — ^‘3 ohms. 

Note.—This method can be employed in the case of alternating 
current systems at work by placing a few cells of a battery in 
place of (9') and using a galvaiiometer that will not indicate 
alternating currents. The mode of procedure is then the same 
as before. 























ELEGTIUOAL ENGINEERING TESTING 


123 


(47) Measurement of Insulation Resistance 
and detection of faulty Telegraph Insu¬ 
lators. 

Introduction. —It is of paramount importance that all insulators 
intended for use on telegraph or telephone lines should be tested 
prior to their erection in the circuits. This will at once be 
evident when it is remembered that the insulators, supporting a 
line having an “earth return,” form so many parallel circuits 
between the line and earth, and though the resistance to leakage 
of current of each insulator may be very great, yet their parallel 
resistance to the same may be very considerably less, allowing a 
very appreciable current leakage to go on to earth continuously. 
This, especially in telephone circuits, is very troublesome, causing 
the lines to interfere with one another, and besides this, speaking 
generally now, it gradually wastes away the batteries of cells used 
in working the lines. Again, the insulation resistance of a 
“line,” composed of all very good insulators except one, will be 
lowered by the one single faulty cup to a value less than that of 
the faulty insulator. 

Hence the importance of preventing, by a suitable and timely 
test, the installation of a bad insulator, which is generally found 
to deteriorate rapidly with time. Insulators should be tested, 
preferably before the bolts are inserted in them, and with an 
E.M.F. of from 100 to 300 volts after from 24 to 48 hours’ 
immersion in a suitable manner in water. The value of the 
results of such a test will depend, however, very greatly upon the 
exact method of preparing the insulators and of applying the 
test, and some precautions have to be attended to in order to 
obtain a result which is trustworthy and /air to the insulator 
itself. The minimum insulation resistance which each insulator 
ought to possess depends on the length of the line on which it is 
to be used. In this country, during the worst wet weather 
conditions, it has been found possible to maintain a minimum 
insulation resistance of ^ megohm per mile on air lines, which is 
therefore taken as the minimum maintenance standard. Now 
the number of poles, and therefore insulators per mile of line, 
varies from 20 to 30 according to whether it is a branch or trunk 


124 


ELECTRICAL ENGINEERING TESTING 


line. Hence, allowing the latter number, each insulator should 
have a resistance of at least 6 megohms if the line were only one 
mile long, 60 if 10 miles long, and 600 if 100 miles long, and so 
on, in the worst wet weather. As a matter of fact the resistance 
of a double shed or cup porcelain insulator, if a good one, may 
vary under testing conditions from 500,000 megohms to some¬ 
thing like 4,000,000 megohms. 

Apparatus. —Shallow trough T lead-lined inside, to which is 
attached the terminal N making contact with the lead; insulators 
to be tested etc., only these two being shown; battery B 

capable of giving 100 to 300 volts E.M.F. and consisting of 
either Daniells, secondary or other convenient cells; very sen¬ 
sitive high resistance reflecting galvanometer G with its shunts 
(S ); high insulation two-way key K (p. 586); high insulation 
test rod t connected to AT by a well-insulated length of gutta¬ 
percha covered wire; standard megohm r. 

N.B.—In this and all other similar high resistance tests, the 
connections should be in mid air as much as possible, and all 
insulating material quite clean and free from dust and finger¬ 
marks. 

Assuming all the insulators to be perfectly clean on the inside 
and outside of their sheds, there are two modes of procedure, in 
each of which they are immersed in the trough T with the water 
up to within half-an-inch of the lips of their outside walls. 
Thus— 

{a) The whole of the inside of each insulator kept clean and 
dry, the resistance then from bolt (5) to water being very 
approximately their insulation resistance under working line 
conditions. This is usually so high that the galvanometer is not 
sensitive enough to indicate the extremely small current passing 
unless the insulator is actually deflective. If it is all right the 
only way usually to obtain a deflection is to test from 100 to 200 
in parallel at one time. 

(6) The inside of the shed or sheds carefully filled with water 
by means of a pipette to within half-inch of the lips, the resistance 
now from bolt {h) to outside water showing whether the insulator 
is faulty through the leakage current passing through its sub¬ 
stance, assuming the unimmersed lips to be clea^i and dry. The 


ELECTRICAL ENGINEERING TESTING 


125 


insulator should be discarded if under these conditions it does not 

CO 



show 600 megohms at least, preferably 2000, which is the minimum 
in some telegraph services. 








































































































































126 


ELEGTRIGAL ENGINEERING TESTING 


In the following test we shall adopt the latter mode of testing, 
since the former is in most cases impracticable. 

Observations. —(1) Carefully clean the porcelain or earthen¬ 
ware portion of each insulator, especially the lips, with clean 
cold water. Then dry the lips merely in a place free from dust, 
and place them with their bolts pointing upwards (Fig. 49) in the 
tank T, packing up the smaller ones so that all the lips are at 
about the same level, but not touching one another. 

(2) Now pour water into T to within half-inch of the lips and 
into the cups, by means of a pipette, to the same level. Then 
allow the insulators to soak for about forty-eight hours (in a 
place free from dust), so that the water will percolate or soak 
into any cracks or flaws in the mass of the earthenware. 

(3) Before testing, hold a hot iron close over the rims 
of each insulator for a short time to dry off any moisture, as 
it is necessary that these parts should be quite dry. For this 
reason avoid testing on a damp day, unless the air of the room 
is dry. 

(4) Connect up as shown in Fig. 49. Test the insulation 
of the connections between t and h before each set of observa¬ 
tions. This will be satisfactory if G does not deflect when the 
connecting wire is supported in mid-air when t is held in the 
hand. 

(5) With the galvanometer at zero and the -g-l^^th shunt in, 
close Kl, so as to bring r into circuit, and note the steady 
deflection id), then close K 2 (opening Jf!) and touch the bolt of 
each insulator successively with t, noting the deflections D in 
each case if any. 

(6) Repeat 4 and 5 for the and no shunt if possible. 

(7) In the case of the double-shed insulators, such as 
(Fig. 49), touch the water between the sheds with t, thus ob¬ 
taining the resistance of the outer shed. Next connect metal¬ 
lically the water between the sheds, and also that outside, and 
touch the bolt with thus getting the resistance of the inner 
shed. 

(8) The approximate resistance of all those insulators which 
show an extremely high insulation can bo obtained by parallel¬ 
ing them or joining their bolts together metallically, and then 


ELECTRICAL ENGINEERING TESTING 


127 


taking a reading (as in 6), first with the of the battery to T, 
and second with the — to Ty the insulators being discharged 
in between the two reversals. Calculate the average of each 
insulator from the combined or parallel resistance so obtained. 

(9) Employ the formula given below, or its modification, and 
tabulate your results as follows— 

Name . . . Date . . . 


Galvanometer Resist. G Ohms at... * C. Internal Battery Resist. = ... Ohms. 


Insulator 

tested. 

After 

hour’s 

inimersion. 

Distance 
of Water 
from Lip. 

Temp. 

of 

Room. 

E.M.F. 

used. 

Shunt 

-S. 

Deflection. 

Knowm 

Resist¬ 

ance 

(r). 

Insuln. 

Resist¬ 

ance 

R. 

known 

d. 

unknown 

D. 












+ + +|)+g}. 

If .S' = ^ (say), then = A- or (1 +£) = 10. 

G 

If no shunt is used S = oz and — = 0. 


(48) Measurement of the Insulation Resist¬ 
ance of Storage Batteries. 

Introduction. —In the erection of a storage battery, provision 
is made for the efficient insulation of every cell composing the 
battery, from earth, by supporting each on suitable insulators, 
and keeping the cells from touching one amother. Notwithstand¬ 
ing these precautions, leakage of current, from various causes, 
may go on in a greater or less degree. It may occur in different 
parts of the battery, or in the leads running from the battery, 
and the resistance opposing the leak may, and usually does, have 
different values at different parts of the battery; in other words, 
the partial “ earths ” at the various points are of unequal resist¬ 
ance, which prevents the insulation resistance being obtained by 
an ohmmeter or other portable testing set. 

The difficulties thus met with are got over by employing the 
present method due to Mr. E. S. Jacob, and which at once gives 
the joint resistance of all the earths at whatever points of the 
battery they may be located. 























128 


ELEGTBIGAL ENGINEERING TESTING 


Apparatus. —Sensitive high resistance galvanometer (?; ad¬ 
justable known resistance box (r); battery to be tested, 

there being any number of cells between the two B^ and B 2 
shown. Key K of very good insulation (p. 586), battery stands, 
or equivalent earth E. 

N.B.—Earths or partial ones, i. e. leaks, may be assumed to 



occur at points P Pi . . . Pq, etc., and r 7?^ . . . to be the re¬ 
spective resistance of such earths. 

Observations. —(1) Connect the key /iT, galvanometer G, and 
resistance box (r) to the battery system at any point P, as shown 
in Fig. 50, E being earth, i. e. the nearest gas- or water-pipe, and 
adjust the galvanometer needle to zero. 

Note. —Choose the point P, so that a conveniently large deflec¬ 
tion is obtained on G, for it will be found that there is a certain 
point of minimum potential from which it increases on either 
side as the contact is moved one way or the other. 

(2) With r = 0, press K and note the current Gq and, for refer¬ 
ence only, the relative position of the point of contact P on the 
system. 


















































ELECTBIGAL ENGINEERING TESTING 


129 


(3) With everything the same as in 2, adjust (r) so that on 
pressing K the new current on G is about J Gq. Note the 
resistance r and this deflection or current G . 

r 

(4) Repeat 1-3 for two or three different values of r. 

(5) Repeat 1-4 for two or three different points of contact P, 

(6) Calculate the insulation resistance of the whole battery 
from the relation— 


E = (f7 + ^) ~ 

Gq - Gf. 

and tabulate as follows— 


ohms, 


Name . . . Date . . , 

Battery: Type . . . Containing boxes of . . . No. of cells . . . 

Galvanometer Resistance g = . . . Oliins @ . . . ® C. 


Relative 
position of P. 

Value of 
r Ohms. 

Current 

^0 

Current 

Cr 

Insulation 

Resistance 

R Oliins. 







• Inference. —Prove the relation given in 6, and state any 
assumptions made in obtaining it. On what does the accuracy of 
the test depend ? 


(49) Insulation Resistance of Dynamos and 

Motors. 

Introduction.—Since the total amount of leakage from a system 
of electrical distribution depends on the number of appliances 
connected to the system, which represent so many parallel paths 
for leakage currents to take, it is of great importance to increase 
the resistance in these paths, and so diminish, as much as possible, 
such currents. This, in other words, means that the insulation 
resistance of all appliances on the system should be as high as 
possible. Hence it becomes of importance to carefully measure 
the insulation resistance of motors and dynamos, both in course 
of manufacture and when installed before running is commenced. 

The Institution of Electrical Engineers recommends that the 
leakage current should not be allowed to exceed part of 

the maximum current in the system at full load. As a general 
rule the total insulation resistance of all the insulated parts of a 

dynamo or motor, joined together, from the frame of the machine, 

K 













130 


ELECTRICAL ENGINEERING TESTING 


i. e. earth, should be at least equal to that of the rest of the 
circuit which it has to run on, but preferably much greater. The 
present test is a simple and convenient one for obtaining the 
insulation resistance of, say, a dynamo, and is as follows— 

Observations. —By means of a high resistance voltmeter of 
resistance ohms measure the P.D. V at the terminals of the 
machine when running at normal voltage not connected to any 
circuit. Also measure the P.D.s Fj and between the + and 
— brushes and the frame of the dynamo. Then we shall 
have— 

Total insulation resistance of the machine 


R = 


V2) 

y, + F2 


R^ ohms. 


(50) Localization of Faults in Electric 
Mains. (Murray’s Loop Method.) 

Introduction. —In the preceding pages, some of the best and 
most important methods of measuring the insulation resistance 
of electric mains have been dealt with, but the localization of the 
position of any prominent and serious fault in such is a matter of 
equal, if not greater, importance. The insulation of the cable 
may or may not have broken down at the fault, but obviously in 
either case, since the opening up of the duct in which the cables 
are laid is often a serious matter in a busy thoroughfare, it is 
important to be able to localize the position of the fault to within 
a very few feet. 

There are two important methods or systems of localizing 
faults, each comprising modifications of the principles employed, 
viz.— 

(^) “ Loop ” methods. (B) “ Fall of Potential” methods. 

The latter, though extremely simple, are less easy of application 
than the former, and must be applied so as to suit the conditions 
of the particular fault. Loop methods are much more generally 
applicable, and are preferable for the following reasons— 

(1) They are “null ” or “zero” methods. 

(2) They are easier to work. 

(3) The accuracy is not affected by variations of the fault 
resistance. 



ELECTRICAL ENGINEERING TESTING 


131 


(4) The test is performed in the same way for mains of any 
section and whatever the fault resistance. 

We shall therefore restrict ourselves to one of the best of 
these loop methods, but before taking it in detail there are a few 
remarks which must be made. 

Loop methods require a continuous circuit of main or cable 
from the place where the testing instruments are, through the 
faulty portion and back again, which is termed the loo2). The 
two portions of cable between fault and instruments should 
have good insulation resistance, but need not have the same 
sectional area throughout. Referring to Fig. 51, let MSN be a 
cable, the two ends of which cannot be brought closer to the 
terminals A and (7 of a Wheatstone Bridge than the points M 
and N respectively. 

Let there be a fault at some point P between the core of the 
cable at P and the earth E, the resistance to earth being much 
less than that at any other point. This is called the fault resist¬ 
ance, and may be represented by r. 

The fault might be on the “ outer ” of a concentric main, in 
which case the “inner” and “outer” conductors would be care¬ 
fully joined at the further end S, so as to form a complete circuit 
or loop. 

In fact, in practice we should either be dealing with this case 
of a concentric main or with a fault on one of a pair of separate 
mains. 

In either case the loop would be formed by making the lest 
possible joint between the two ends furthest from the testing 
point, by disconnecting them from any terminals to which they 
might be clamped. Now it is obvious that the cable ends M and 
N cannot be clamped under the terminals of the bridge, owing to 
their size and to them terminating in a position in which it would 
be impossible to have the testing apparatus. Smaller connections 
AM and (7iV"must therefore be used and also allowed for in working 
out the results of observations, and this is done as follows— 

Let Dj i >2 = distances from M and N respectively to the looping 
point, 

and = from A to M and from G io N respectively, 

s = sectional area of either conductor of the main MSN^ 

and ^ 2 =sectional area of the connections AM and CN 
respectively. 



132 


ELECTRICAL ENGINEERING TESTING 


Then, since resistance is directly cc 1^^—, we see that AM 

section 

has the same resistance as, or is equivalent to a length of the main 
MSN = —. d-, and ON to a length = —.cL. 

Si S2 

Hence the bridge will behave as if the resistance between C and P 
(which = resistance of 6'iV^+resistance of NP) was that = to a 

s 

length of the main cable = 7 ) 2 + — • <^ 2 * 

The whole length of circuit L between A and C is— 

. *. 7/ = + — • d-^ 4* — * d^ 7^2" 

Sj S.2 

The expression or value for L may very greatly be simplified 
by making 7)^ = and also d^ = ^ 2 , which latter can easily be 
done, and by choosing single wires for the connections AM and CN 
of the same gauges as each wire of the strand forming the main 
MSN. Thus if the main was yj in size, use for the 
connections. 

If this is done and n = N°. of wires in the strand of either 
conductor of the main, 

then L — 27> + 2. n 7 = 2 (7) + 7.) 

The lengths of AM and (7iV, though =, should be as short as 
possible, so that their resistance is not large compared with that 
of the main cable. 

Apparatus.— Post-office or other pattern Wheatstone Bridge ; 
sensitive galvanometer G (p. 569); battery B of Leclanche cells; 
fuse F\ resistance 7?; suitable connections AM and CN and 
main to be tested. 

Note.—A few secondary or primary cells may form the 
battery By and if the fault resistance r is anything like 1000 
ohms, a battery giving 100 volts might be used. The fuse F is 
to prevent the bridge coils being fused up should r break down 
unexpectedly, and it can be dispensed with if a primary battery 
is used. It is as well to earth the battery, if not a primary one, 
through a resistance 7? of a few ohms. 

The more sensitive G is, the greater the accuracy of the test, 
and quite small wires may be used to connect G and B up, as 
their resistances do not affect the accuracy of the test. 



ELECTRICAL ENGINEERING TESTING 


133 


Observations. —( 1 ) Connect up as shown in Fig. 51, and adjust 
the galvanometer G nearly to zero. See that a very fine fuse 
is inserted in E, if necessary at all, and make fairly good earths 
at EE. Good soldered joints at J/, N and the looping point are 
practically indispensable, as well as good tight clean contacts at 
A and C. Inattention to these points will vitiate the results. 

( 2 ) Plug up 9*1 = 0 and unplug 9 - 2 = 1000, with, say, the Infinity 
plug out in r^. 



Fig. 51. 


(3) Press first and then tap for an instant^ observing 
which way G deflects for 9*3 so great. 

If on inserting “ Inf.” G defects the other way on manipulating 
/fj and as before, then a balance can be obtained by adjusting 
rg so that G does not deflect on pressing /iTj and then K<^. Note 
the value of 9 * 9 , 9 * 3 . If no balance can be found, the fault P is 
very close to M or N when the connections to A and G should 
be interchanged. 

(4) If the balance appears to be insensitive 9 * 2=100 or 10 may 
be tried and (obs. 3) above repeated. 

(5) Calculate the equivalent distance of the fault P from the 
terminal G by means of the relation 

Z= — 

^2 + ^'3 

and tabulate your results in a convenient way. 


N.B.—The distance of P from N therefore = I — — d^. 

( 6 ) Interchange the connections to A and G and repeat obs. 
2 - 5 , and if the result now given is close to the previous one, take 






















134 ELEGTBIGAL ENGINEERING TESTING 

the mean of the two as the correct one. If they differ consider¬ 
ably, the contacts are bad and must be improved. 

In this latter test the distance of P from A is given by 

I = J^-L. 

^2 + 


(51) Calibration of Speed Indicators. 

Introduction. —There are many different forms of speed indi¬ 
cators or tachometers, as they are frequently termed, some of 
which vary in accuracy with the time for which they are in use. 
It thus becomes of importance to standardize such instruments 
at frequent intervals when accuracy is required. This can 
be done either by comparison with an accurately calibrated 
tachometer specially reserved for this purpose only, or by taking 
simultaneous readings on the instrument to be checked and 
time readings on a speed-counter. This latter method, being 
the more generally applicable of the two, will be the one here 
considered. 

Apparatus. —Tachometer to be tested ; speed- or revolution- 
counter ; stop-watch if available, or in lieu of this an ordinary 
watch with a “ seconds-hand." 

Observations. —(1) Arrange the tachometer so that it can be 
driven by some machine at a variety of speeds, each of which 
may be maintained constant for at least one minute. This driving 
machine must have a turned centre in one end of its shaft into 
which can be pressed the centre of the spindle of the revolution- 
counter. 

(2) Drive the tachometer at the lowest convenient speed 
readable on its scale, and take the dial readings (Dj) of the revolu¬ 
tion-counter. When this speed is constant (as will be observable 
by the instrument itself) insert the counter in the end of the 
driving-shaft at a noted instant of time, using a light pressure. 
Quickly take the counter away, the instant one minute has elapsed, 
and note the readings (D.J of its dials. Then == speed 

in revolutions per minute. 



ELECTRICAL ENGINEERING TESTING 


135 


(3) Kepeat 2 twice or three times at the same speed, and 
record the mean. 

(4) Repeat 2 and 3 for some eight or ten readings on the 
tachometer, rising by about equal increments to the maximum. 

(5) Check some of the readings obtained in 2-4 by taking 
observations over two minutes. 

(6) If the tachometer is adjustable, alter it so that its indica¬ 
tions give the correct speed in revolutions per minute. If unad- 
justable, plot a calibration curve having values of true speed as 
abscissae and tachometer readings as ordinates. 


(52) Measurement of Rotational Speed by 
the Stroboscopic Fork. 

This method has been in use for measuring the speed of 
generators and motors for many years. See papers by Dr. 
C. Y. Drysdale—(1) on “Stroboscopy”; read at the Optical 
Society, London, in 1905, and reprinted in The Optician and 
Photographic Trades Review^ Dec. 8 and 15, 1905. (2) “Accurate 
Speed, Frequency, and Acceleration Measurements,” Electrical 
Review^ London, Sept. 7 and 14, 1906. Previous to that it 
had been used by scientists, e. g. to measure the speed of a small 
driving motor to within of 1% in the Lorenz apparatus for 
determining the absolute value of the ohm. 

The stroboscope 
used comprises es- ^ ^ 

sentially—(1) either 
a hand vibrated, or 5 ,"’ 

(preferably) an elec¬ 
tro-magnetic ally 
vibrated tuning fork 

fitted with two shutters, each containing a narrow slit, and 
(2) a discoidal target of observation. 

The stroboscopic fork unmounted is indicated diagrammatic- 
ally in Fig. 52, and that used by Dr. Drysdale consists of a steel 
tuning fork, the limbs LL of which are approximately 12" long 
X wide X thick, and are supported by a tail-rod T 




T—r 

I I 
I I 
I > 



Fig. 52. 



















136 


ELECTRICAL ENGINEERING TESTING 


clamped in a suitable frame or stand (not shown). To the 
extremities of LL are screwed two light and thin metal strips or 
flat shutters ^ t¥') plane of vibration, each 

containing a narrow slit S (|" long) parallel to the length of LL, 
These slits are exactly opposite to one another when the fork 
is at rest, so that in this condition the eye can see through 
both slits. When hand vibrated the fork, while being held 
in the observer’s hand, is excited into vibration at intervals 
by mechanical impulses derived from a light blow on the 
knee. 

When electro-magnetically operated, the “make and break” 
principle used in the electric trembling bell is adopted—an iron- 
cored coil G being mounted between LL on a support (not 
shown) adjustable, for convenience, in a direction parallel 
to LL, 

This actuating coil C is energized from one or two dry cells 
through a “ contact breaker,” formed by two platinum-tipped con¬ 
tacts—one on a fixed support, the other on a very light flat 
spring fixed to one of the limbs L, Now when the fork is 
excited into free vibration, the line of vision through the slits S 
is interrupted by the vibrating shutters except during a 

very brief interval, once in each half-cycle, or twice in each 
complete cycle of the vibration, at the moment when the slits S 
pass each other, moving rapidly in opposite directions. If, then, 
an object viewed through the slits S is rotating so that con¬ 
secutive images conve 3 ^ed to the eye are similar and symmetrical, 
that object will appear to the eye to be continuous and stationary, 
although actually rotating at a high speed. 

Further, a certain cyclic departure from absolute similarity 
and symmetry in the successive images seen by the eye will 
make the object appear as if it was rotating. If the object 
appears at a standstill when seen through its speed must be 
constant, and must bear a simple proportion to that of the 
vibration of the fork. Therefore, since the rate of vibration of 
a tuning fork is always very constant at all times, and almost 
independent of change of temperature (varying only about 
0 -01% per deg. cent.), it can be determined once for all with 
great accuracy, and hence also any speed by comparison can be 
measured with like accuracy 


ELECTRICAL ENGINEERING TESTING 


137 


If a fork vibrates with a frequency of 50 cycles per sec., the 
eye will receive 100 impressions of the object through the 
slit S per second, and will have 6000 peeps per minute. The 
geometrical form of the rotating object viewed through the slit 
will, in conjunction with the rate of vibration of the fork, 
decide the speeds at whicli the object will appear at a standstill. 
These speeds might, for example, be 100, 200, 300, etc., revs, 
per min., and the simple fork shown in Fig. 52 can only serve 
to measure speeds intermediate between tliese by enabling the 
eye to count the apparent revs, made by the object in one minute 
by a watch. For example, if the speed at which the object— 
viewed through the slit appeared stationary—was 1500 revs, per 
min., and afterwards the object appeared to be rotating in the 
direction of motion at 50 revs, per min., then the actual speed 
of the object would be 1550 revs, per min. If this apparent 
rotation of the object was in the opposite direction, the actual 
speed would be 1450 revs, per min. 

The determination in this manner of speeds intermediate 
between the two nearest standstill valves, coupled with the 
necessity for such being constant, is a disadvantage in the 
stroboscopic method, for it is obviously preferable to be able to 
bring the object to an apparent standstill at any speed. At 
least two methods of doing this have been devised: one due to 
Dr. Drysdale employs a special arrangement of a conical roller 
with stroboscopic disc, as described in his paper, but which lacks 
portability. In another method, designed for portability by 
Messrs. A. E. Kennelly and S. E. Whiting, and described by 
them in a paper read at the Twenty-fifth Annual Convention of 
the Amer. Inst.E.E., Atlantic City, H.J., June 29—July 2, 1908, 
the stroboscopic fork is adjustable continuously in its rate of 
vibration through a range of about 5% above and below its 
mean value without sensibly disturbing its motion. This is 
done by a pair of sliding weights, which are moved friction tight 
along about of the limbs of the fork, gradually from one 
position to another, by. a pair of strings, normally slack, 
passing over guide-pulleys, and vibrating with the fork. The 
form used in this design has limbs 18" long X 1" wide X iV 
thick, and weighs by itself almost 2 lbs. The object in using 
50 long a fork was to obtain a low frequency vibration or fork- 


138 


ELEGTRIGAL ENGINEERING TESTING 


speed comparable with the speeds ordinarily met with in rotating 
machinery. The fork has a mean speed giving 1900 peeps per 
min., and carried on its extremities a pair of thin sheet steel 
shutters with slits 0*59" long X 0*008" wide. Adjustments 
are provided for aligning these slits, and for tuning the fork to 
the required normal frequency. It is operated by an electro¬ 
magnet, the coil of which is wound with 525 turns of No. 27 
S.W.G. enamel insulated copper wire (enamelled to No. 26 
S.W.G.), having a resistance of about 3|- ohms, and requiring 
an average current of about 0*15 amp. to operate the fork 
through a form of contact breaker already referred to. 

The amplitude of the vibration of the fork limbs is about 
at the slits, each side of their position of rest, giving a maximum 
cyclic velocity of about 1 foot per sec. at normal fork speed. 
The relative velocity of the slits is thus about 2 feet per sec., and 
the duration of each peep through the slits will be ^ 

of a min. Hence an object rotating at 1800 revs. 


sec. 180000 

per min. will only move through = tou ^ revolution 

during each peep, and if it appears at a standstill through the 
slits, 1 rev. per min. more or less, i.e. a variation of ~ of 1% 
in the speed will cause the object to make 1 rev. per min. either 
way. 

The Target —hitherto called the rotating object^ may most 

conveniently consist of a thin disc 
(between 9" and 10" in diam.) of 
slieet metal or thick cardboard 
fixed concentrically—in any con¬ 
venient and satisfactory way—to 
the end of the shaft whose speed 
is to be measured. If neither end 
of the shaft system is available, 
nor any portion of the end of the 
machine suitable for use as a tar¬ 
get, it may be possible to mount 
the special artificial target on a 
subsidiary spindle with pulley, and 
drive this by a light supple tape belt at a definite speed ratio 
from some part of the machine under test. 

From many trials with different kinds of targets, it has been 



Fig. 53. 




ELECTRICAL ENGINEERING TESTING 


139 


found that a pure white pattern on a deep black grounding, and 
of about the size above-mentioned, is quite satisfactory. The 
pattern (Fig. 53) comprises: a square, a pentagon, a hexagon, 
a 14-point star, and an 18-point star, all concentric with one 
another, and with the disc and shaft, with a radial bar inside 
the square for enabling the apparent revolutions between 
intermediate standstill speeds to be counted easily. 

If P = number of positions of symmetry per revolution of 
any individual pattern on the target. 
n = revs, per min. of the target. 

N = any whole number or reciprocal of a whole number, 
and p = number of peeps per min. given by the fork’s rate of 
vibration. 

Then the particular pattern will stand still when viewed 
through the slits if 


N = 


F. 


n 


P 


For example, the square or 4-pointed star has P = 4 positions 
of symmetry per revolution. 

If a given fork provides p = 3600 peeps per min.. 

Then the square will appear to stand still for actual speeds of— 

450 revs, per min. at which N = — == — ^ = I 

p 

the reciprocal of a whole number, 


n 


3600 


and n = 1800 revs, per" min. at which N = 


Pn _ 4 X 1800 
V ~ 3600 


2 


a whole number. 

In the first case the square will move through half of a position 
of symmetry, and in the second case through two positions of 
symmetry between successive peeps. 

The useful possibilities obtainable with the above target may 
be conveniently seen in Table VI. 

The higher the actual speed of rotation, the greater must be 
the rate of vibration or frequency of the slits and number of 
peeps per min., and the narrower must the slits be, otherwise 
the number of positions of symmetry P must be smaller to give 
an increase of pitch. 

The accuracy of this method of measuring speed is very high, 
and of the order of about 1 part in 10,000. 







140 


ELEGTRIGAL ENGINEERING TESTING 


Table VI.—Actual speeds in revs. per. min. at which each pattern of the Target (Fig. 53) will appear 
to stand still when viewed through the slits of a Fork giving 1800 peeps I'er minute. 


Square. 

Pentagon. 

Hexagon. 

14-Pointcd 

Star. 

F = 14. 

18-Pointed 

Star. 

F = 18. 

F = 4 or p 

F = 8 or 1 

F = 5 or i 

F = 10 or 

F = 6 or 

F = 12 or 

Image 
stationary 
at every 
450r.p.m. = 
i Synchr. 
Speed. 

Image 
appearing 
doubled, 
but less 
clear at 
Intermedi¬ 
ate Speeds 
or every 
225 r.p.m. = 
i Synchr. 

Speed. 

Image 
stationary 
at every 
360 r.p.m. = 
1 Synchr. 
Speed. 

Image 
appearing 
doubled, but 
less clear at 
Intermediate 
Speed or 
every 

180 r.p.m. = 
Synchr. 
Speed. 

Image 
stationary 
at every 
300 r p.m. = 
\ Synchr. 
Speed. 

Image 
appearing 
doubled, but 
less clear at 
Intermediate 
Speed or 
every 

150 r.p.m. = 
Synchr. 
Speed. 

Image 
stationary 
at every 
128-6 
r.p.m. = 

^ Synchr. 
Speed. 

Image 
stationary 
at every 
100 r.p.m. = 
Synchr. 
Speed. 

— 

225 

- 

180 


150 

128 6 

100 

450 

450 

360 

360 

300 

300 

257-2 

200 

— 

675 

— 

540 

-- 

450 

385-8 

300 

900 

900 

720 

720 

600 

600 

514-4 

400 

— 

1125 

— 

900 

— 

.... 

643-0 

500 

1350 

1360 

lOSO 

1080 

— 

750 

771-6 

600 

— 

1575 

— 

1260 

900 

900 

900-2 

700 

1800 

1800 

1440 

• 1440 

— 

1050 

1028-8 

800 

— 

2025 

— 

1620 

1200 

1200 

1157-4 

900 

2250 

2250 

1800 

1800 

— 

1350 

1286-0 

1000 

— 

2475 

— 

1980 

1500 

1500 

1414-6 

1100 

2700 

2700 

2160 

2160 

— 

1650 

1543-2 

1200 

— 

2925 

— 

2340 

1800 

1800 

1671*8 

1300 

3150 

3150 

2520 

2520 

— 

1950 

1800-4 

1400 

•- 

3375 

— 

2700 

2100 

2100 

1929-0 

1500 

3600 

3600 

2880 

2880 

— 

2250 

2057-6 

1600 

— 

3825 

— 

3060 

2400 

2400 

2186-2 

1700 

4050 

4050 

3240 

3240 

■— 

2550 

2314-8 

1800 

— 

4275 

— 

3420 

27C0 

2700 

2443-4 

1900 

4500 

4500 

3600 

3600 

— 

2850 

2572 

2000 

1 




8000 

3( 00 




(53) Relation between Speed and E.M.F. in 
“Separately Excited,” “Shunt,” and 
“ Compound Wound ” Direct Current 
Dynamos. 

Introduction. —The following tests are arranged with the 
object of investigating the way in which the terminal E.M.F. 
of the various types of dynamos varies with the speed when 
the machine is delivering no extei’nal current. The mode of 
procedure is exactly the same for each type of machine, and 
consequently this will be given in one concrete instance only, 
and merely referred to afterwards for the others. In all cases 
the terminals T 2 of the machine are the ones to which the 
external circuit would be directly connected. Prior to starting 
see that all lubricating cups in use contain oil, and feed properly 

































BLEGTRIGAL ENGINEERING TESTING 


141 


but very slowly ; also that the commutators are smooth and clean, 
and the brushes properly trimmed. 

The performance of a “magneto” machine would be 
exactly similar to that of (A) below, and the “ series ” machine 
approximately the same also, providing the current through the 
series machine be kept constant by varying the circuit resistance 
as the speed varies. 

Apparatus. —Dynamo D to be tested; tachometer; voltmeter 
(F) capable of reading sufficiently high. 

{A) Separately Excited Dynamo. 

Observations.— (1) Connect up V to and the exciting 

coils to some outside source of E.M.F. through an ammeter 
(a), and rheostat (J?), then start D. Adjust {11) so as to obtain 
a current which will give ^ max. excitation (to be kept constant). 

(2) Adjust the speed so as to obtain the lowest readable scale 
reading of F. Note this and the speed. 

(3) Kaise the speed so as to get about ten different values of F, 
rising by about = increments to the max., and note the speed at each. 

(4) Repeat 3 for a similar descending set of readings. 

(5) Repeat 3 and 4 for such a current through the field coils 
as will give max. excitation, and tabulate your results in the 
following general form. 


Type of Dynamo 
tested. 

Speed in Revs, 
per min. 

Separate Exciting 
Currents (if any) 
a. 

Terminal B.M.F.s. 

Ascending 

V. 

Descending 

V. 







(R) Shunt Dynamo. 

Observations.—(1) Connect up F to T^^ and start the 
dynamo. 

(2) Repeat observation (2—4 A), and tabulate in the form 
shown above. 

(C) Compound Wound Dynamo (Long Shunt). 

Observations. —Repeat those for B above. 

Plot curves for the tests on each type of machine in A, R and 
C above, having E.M.F. as ordinates and speed as abscissae. 

Inferences. —Compare the above curves and results, and state 
clearly all that you can infer from them. 














142 


ELECTlUGAL ENGINEERING TESTING 


Characteristics of Dynamo Machines. 

Introductory Eemarks. —There are two great classes of 
electrical generators for converting mechanical into electrical 
energy—namely, (1) those which supply continuous current, i. e. 
current which flows in one direction only round the circuit, and 
which are otherwise styled direct current dynamos; (2) those 
which supply alternating current, i. e. current that reverses its 
direction throughout the whole circuit many times a second, 
and which are usually styled alternators. As therefore the 
supply of electrical energy to any appreciable extent invari¬ 
ably assumes one or other of the above forms of current, a 
study of the behaviour of the machines that supply it becomes 
of paramount necessity. Restricting our considerations first of 
all to the former of the above-mentioned classes, it may be 
remarked that it is composed of a great variety of forms, the 
performance of which depending practically entirely on the 
method employed in winding their field magnets, in other words, 
as to whether the whole, a fraction, or a combination of this 
whole and the fraction of the whole current generated by the 
machine is utilized in magnetizing their field magnets. Direct 
current dynamos in general may consequently be subdivided into 
the following five distinctive types according to the winding of 
their field magnet coils— 

(a) Magneto machine. 

(5) Separately excited machine, 

(c) Series machine. 

{d) Shunt machine. 

(e) Compound machine. 

All these types are used in practice, especially the last three, 
which form by far the greater proportion of direct current 
machines in use throughout the world. Only by a minute study 
of the performance and action of each type can it be seen which 
of them is the best suited for any particular purpose. 

The current that any particular dynamo will send through a 
given external circuit connected to its terminals will obey Ohm’s 
Law, and will depend on the E.M.F. of the machine, as well as 
on this external resistance. 

If therefore the machine be run at a constant speed, and the 


ELECTRICAL ENGINEERING TESTING 


143 


circuit resistance R varied by suitable steps (say), both the ter¬ 
minal P.D. of the machine (T) and the current A will vary. 
Assuming that V and A are noted simultaneously for each alter¬ 
ation of R we can plot a curve having values of V, measured 
along the ordinates, and A along the abscissae, these axes being 
rectangular. Such a curve is commonly called the ‘'external 
Characteristic^^ of the dynamo, and by means of it many valuable 
and practical details can at once be deduced. 

In fact, the function of a Characteristic in relation to a dynamo 
is extremely analogous to that of an “ Indicator diagram ” with 
an engine. In the former, not merely can the qualities and per¬ 
formance when working be seen, but also the H.P. at which it 
works or could most economically work at, and many defects in 
the design—such as the sufficiency of the field magnet field, the 
degree of saturation of the magnets, the demagnetizing action of 
the armature on the field, etc., etc. The Characteristic of a 
dynamo is therefore much more important to the electrical 
engineer than the author ventures to think is generally supposed. 

(54) Determination of the ‘‘ External Charac¬ 
teristic’' of a Magneto Dynamo. 

Introduction.—The present type of machine under consider¬ 
ation has a somewhat extensive field of use, two very important 
applications being for blasting purposes and use with the ohm- 
meter, and indeed in all kinds of work in which a portable E.M.F. 
and small current is desired. 

Another application in the past on a heavier scale was in the 
production of lighthouse search-lights, in which kind of work 
heavy currents are required at a comparatively low E.M.F. 
Considering the machine more in detail, when the armature is 
delivering no current to the external circuit, the terminal 
P.D. (F)=.the total E.M.F. (A) of the dynamo. When, however, 
a current A flows, then V is less than E by an amount depending 
on the armature resistance r ^; for, by Ohm’s Law, 

we have E = A {R-\-r^ 

where R = the resistance of the external circuit; but since it is 
solely the P.D. (F) which drives the current through the external 
resistance, 

V=AR, 

and consequently E == V+Ar„. 


144 ELEGTBIGAL ENGINEERING TESTING 

Thus we see that the total E.M.F. of the dynamo is = the P.D. at 

the terminals + that required to 
send the current A through the 
internal resistance of the arma¬ 
ture. The permanent magnetism 
of the steel magnets is approxi¬ 
mately constant. 

Apparatus. —Magneto dynamo 
D to be tested; voltmeter V; am¬ 
meter A ; switch S ; rheostat R 
(p. 606); tachometer. 

Observations. —(1) Connect up 
as in Fig. 54, and adjust the 
pointers of A and V to zero, if 
necessary. See that all lubricat¬ 
ing cups in use feed slowly and 
properly, and that the commuta¬ 
tor is smooth and clean, and the 
brushes p'operly trimmed. 

(2) Start B up to its normal 
speed, and when this is constant, note the reading of Y. 

(3) R being as large as possible, close S, and take a series of 
about ten different values of current A, rising by about equal 
increments from the smallest to the maximum permissible. Note 
the voltage Y and current A for each, the speed being constant at 
the above value. 

(4) Repeat 2 and 3 for a similar descending set of readings of 
F and A at the same speed. 

(5) Repeat 2-4 for speeds 20% above and 50% below normal 
respectively. 

(6) Measure the armature resistance (?*„) (while warm) by the 
Wheatstone Bridge, or if it is too low, by the “ Potential 
Difference” method (p. 84), or by the ammeter and voltmeter 
method (p. 86), and tabulate your results as follows— 


Name . . . Date . . . 

Magneto Dynamo tested: No. . . . Tj'pe . . . Maker . . . 

Normal Voltage =. . . Amps. =. . . Speed =. . . . va=. . . O'iiiis. . . 


Speed 

Revs, per min. 

Ascending. 

Descending. 

Terminal 
Volts (F). 

Amps. (J). 

Terminal 

Volts (F). 

Amps. (A). 






























ELECTRICAL ENGINEERING TESTING 


145 


(7) Plot the external Characteristic curves of the machine for 
both the ascending and descending readings at each speed on the 
same curve sheet, having values of V as ordinates and A as 
abscissre. 

(8) Deduce from these the total Characteristics of the machine 
by the graphical construction given below, drawing them on the 
same curve sheet {vide p. 3, obs. 8). 

(9) Plot the horse-power curves (see below) to the same axes. 
Inferences. —State clearly all that you can infer from your 

experimental results. 



Fig. 55. 

Graphical Deduction of the Total Character¬ 
istic of a Magneto Dynamo from its 
External Characteristic. 

Referring to Fig. 55 : take two co-ordinate axes having their 
origin in the point 0. Let xy be the external Characteristic. 
Take any point Q in the abscissae representing some definite 
current OQ to the scale chosen, and set off on the ordinates 
QP := OQ X volts. 

Join OPy which therefore represents the fall of Potential 
through the armature due to the currents flowing in it, 

or tan. POQ = ra- 


L 









yo/As 


146 


ELECTRICAL ENGINEERING TESTING 


Now take any point a' in ay, and through it draw an ordinate 
cutting OF and OQ in h and a respectively. Set off a'F = ab, and 
U will then be a point on the total GhaTO/Ct&ristic. 

Repeat this construction for eight or ten points, such as a 
along xy, and finally draw the total Characteristic xb rec[uired. 



Fig. 56. 


The performances of the machine, deducible from the above 
diagram, are roughly these— 

If the curves xy and xb' are too far apart, it means that the 
armature resistance is excessive and is causing a large drop of 
terminal voltage. 

Again, if the lower part of xy droops considerably, it shows 
that the armature current is causing a considerable demagnetizing 
action for the larger currents when the angle of lead of the 
brushes is greater. This downward drooping occurs more 
markedly as the field magnets become weaker. 















ELEQTUrCAL ENGINEERING TESTING 


147 


Horse-Power Curves. 

Since the electrical power W in Watts developed by any dynamo 
giving a current A amps, at a P.D. of V volts is— 

Tr=^F (Watts) 
and since 1 E.H.P. = 746 (Watts), 

. *. H.P developed = 

Now manifestly, as the Characteristics of a dynamo are plotted 
to definite scales of volts and amperes, the power developed at 
any point on such a curve will be equal to the product of the co¬ 
ordinates of that point, and can therefore at once be seen. 

If, however, curves of equal H.P. are, at the onset, drawn to 
the axes chosen, the above calculations will be avoided, for if the 
2 H.P. curve cuts the Characteristic at a given point, then the 
power developed by the machine corresponding to the V and A of 
that point will be 2 H.P. 

To determine these H.P. curves : Find several points such that 
the products of their co-ordinates = 746 Watts = 1 H.P. Thus, in 
Fig. 56 we have— 

FQxQA = MNx NW^ GD x Z)(7= 746 Watts = 1 H.P. 
similarlyP7?xi?TF=/i:AS'xAS'^ =MVx F(7= 1492Watts = 2H.P. 
and XZ X Z]V=^ KT x TY = 2238 Watts = 3 H.P. 

Then all points on the curve QND represent 1 H.P. 
and „ „ RSV ,, 2 H.P., and so on. 

Intermediate values of H.P., such as 1*5 H.P., can be got by 
halving the distances between curves QND and RSV, etc. 

These H.P. curves are simply rectangular hyperbola) if equal 
scales are used on the axes, but are distorted hyperbolae if the 
scales are unequal. Instead of H.P. they can equally well be 
drawn to represent kilowatts. 

(55) Determination of the External Charac¬ 
teristic of a Separately Excited Dynamo. 

Introduction. — The type of machine under consideration 
as representing a direct current dynamo has little applica¬ 
tion in practice, but as representing an alternating current 



148 ELEGTBIGAL ENGINEERING TESTING 

machine it has an extremely wide range 
of use. Such, however, we do not 
propose to discuss at this stage. Its 
utility in the first-named use lies in 
the fact that not merely can a far 
more powerful field be produced than 
is possible with any permanent steel 
magnets, which is of great value and, 
in fact, all-important in the genera¬ 
tion of large E.M.F.s, but such a 
machine can be used with perfect safety 
in charging secondary cells, electro¬ 
plating, etc., without the least fear of 
its polarity being changed should the 
back E.M.F. of the cells exceed that of 
the machine. A further advantage may 
be mentioned, viz. that a wide variation 
of E.M.F. can be obtained at any speed by suitably varying the 
exciting current (a) by the rheostat (r). The main disadvantage 
lies in having to provide an independent source of E.M.F. [B) 
for exciting the field magnets {F.M,), 

The total E.M.F. is, as in the case of the magneto dynamo, given 
by the relation E=V-\- Ara, where A — the current flowing in the 
circuit at a terminal P.D. = F and = the armature resistance. 

The external and total Characteristics of the machine are 
found in exactly the same way as for the magneto dynamo, but 
at constant excitation as well as speed, and Fig. 57 shows the 
requisite apparatus symbolically and the connections of the 
same. 





B 



1 


F.M. 



(56) determination of the Internal Charac¬ 
teristic or Curve of Magnetization of a 
Separately Excited Dynamo. 

Introduction. —In any dynamo, the P.D. across the brushes, i.e. 
armature, when no current flows in the external circuit, gives a 
measure, and is proportional to the magnetization of the field 





















ELECTBIGAL ENGINEERING TESTING 


149 


magnets. If therefore the field magnets of any dynamo are separ¬ 
ately excited by different currents from an independent source of 
tlie corresponding voltages across the brushes,/or constomt 
speedy will be approximately proportional to the inductions pro¬ 
duced, if the machine is giving no current. The curve relating 
to exciting currents with E.M.F.s is termed the internal Character¬ 
istic^ and it shows the region of saturation of the magnets, and 
also whether the eddy currents in the armature are producing 
any perceptible demagnetization of the field, and therefore shows 
the efficacy of the lamination of the armature core. 

Apparatus. —Separately excited dynamo D to be tested ; volt¬ 
meter Y\ exciting circuit comprising source of E.M.F. {B ); am¬ 
meter a\ switch K\ rheostat (r) (p. 599); and field coils F.M. 

Observations. —(1) Connect up as in Fig. 57, omitting the main 
current circuit there shown connected to the armature. Adjust 
the instruments to zero. 

(2) Start D up to normal speed and with (r) large, close aS', 
and take about ten different values of exciting current, rising 
by about equal increments from 0 to the maximum, and note the 
corresponding voltage Y at each, the speed being constant 
throughout. 

(3) Repeat 2 for a similar descending set of readings, and 
tabulate your results in a convenient manner. 

(4) Plot the internal Characteristic having values of terminal 
voltage V (cc to field flux) as ordinates, and exciting currents 
(a) (oc to magnetizing force) as abscissce. 

Inferences. —State clearly the meaning of the curve, and any 
inferences which can be drawn from the experimental results. 

(57) Relation between External Current and 
Exciting Current, at Constant Voltage, 
in a Separately Excited Dynamo, at 
Constant Speed. 

Introduction. —If both" the external and exciting currents 
are varied together in such a way that for a constant speed 
the terminal voltage is constant, then the curve showing the 
variation of one current with the other will show the increase 
of excitation that would be necessary to give constant voltage 
for varying external resistance at constant speed. 


150 


ELEGTRIGAL ENGINEERING TESTING 


Apparatus. —Precisely that required for the preceding test, and 
in addition the main circuit comprising ammeter A; rheostat R 
(p. 606); and switch S. 

Observations. —(1) Connect up as in Fig. 57, and adjust the 
instruments to zero. See that all lubricating cups in use feed 
proi)erly. 

(2) Start D up to the normal speed, and with a convenient 
excitation note the voltmeter reading F, which in future is to be 
kept constant as well as the speed. 

(3) With R large close S, and take about ten different load cur¬ 
rents A, rising by about equal increments from 0 to the maxi¬ 
mum allowable, adjusting the exciting current to keep the volts 
constant. Note each pair of corresponding currents. 

(4) Pepeat 3 for a similar descending set of readings. 

(5) Repeat 3 and 4 for a different speed but the same voltage 
by suitably altering the initial excitation, and tabulate in a con¬ 
venient form. 

(6) Plot curves for each speed, for both ascending and descend¬ 
ing readings having values of exciting current as ordinates and 
main currents as abscissje. 

Inferences. —State clearly the practical value of the above tests, 
and explain the form of curve so obtained. 


(58) Determination of the External Charac¬ 
teristic of a Series Wound Dynamo. 

Introduction. —In this type of machine the whole current 
developed at any time is employed for magnetizing the field 
magnets, but these are only wound with a comparatively small 
number of turns of thick wire to carry this main current. 

The series machine is an extremely important one in practice, 
and will be found throughout the world in different parts. It is 
essentially employed as a constant current dynamo, usually develop¬ 
ing a small current at a high E.M.F. The principal application 
is in the lighting of arc lamps in series, and as an electro-motor 
in the various branches of electric traction work. Regarding it 
first of all from a theoretical standpoint, there is only one circuit 
in operation, and hence but one current, in the case of a series 


ELEGTRIGAL ENGINEERING TESTING 


151 


dynamo running an external circuit. Thus when this is broken 
there is no E.M.F., and the maximum E.M.F. is only obtained 
when close on the full load current is flowing. 

The series dynamo possesses many peculiarities, the nature and 
effect of which, on practical working, it is all-important to be 
cognizant of. This investigation is best solved by obtaining the 
Characteristic of the machine and studying it. 

If Tg and Ta be the resistance of the series coils and armature 
respectively, and R that of the external circuit, then when the 
voltage across the terminals is V, the total E.M.F. generated 

E = A {R r, -h Q'a) 

where A = the current in the circuit; but V — AR ; hence 

E = V + A 

Apparatus. —Series dynamo D to be tested; voltmeter F; 
ammeter A ; switch B ; low resistance (variable) rheostat R 
(p. 606); tachometer. EM represents the fleld magnet (series) 
coils ; TT represent the terminals of the dynamo. 

Observations. —(1) Connect up as in Fig. 58, and adjust the 
instruments to zero, if necessary. See that all lubricating cups in 
use feed slowly and pro- 
perly, and that the brushes ‘ 

are properly trimmed and 
the commutator smooth and 
clean. 

(2) Start D up to its 
normal speed, and with S 
open note the reading on 

V (if any). 

(3) With R large, close 
S, and take about ten 
values of current A {at 

constant speed), rising by 
about equal increments from 0 to the maximum permissible, and 

note the corresponding voltage V at each. 

Uote.— In starting, care must be taken not to decrease R too 
quickly, as the machine might suddenly build up on its residual 
magnetism, and a large rush of current ensue. 

(4) Kepeat 2 and 3 for a similar descending set of readings of 

V and A. 












152 


ELECTRICAL ENGINEERING TESTING 


(5) Repeat 2—4 for speeds 20 % above and 50 % below normal 
respectively. 

(6) Measure, by means of the Wheatstone Bridge, the resistance 
- Tg of the series coils, of the armature, and r, + of the whole 

machine while warm, supposing it to be not too small, and tabulate 
your results as follows — 

Name . . . Date . . . 


Series Dynamo tested : No. . . . Type . . . Maker . . (ro -j-»■«) = • • 

Normal Voltage = . . . Amps. = . . . Speed = . . . Resistance ra = . . . r« = . . 


Speed 

Revs, per min. 

Ascending. 

Descending. 

Resistance of 
External Circuit 

^ohms. 
mean A 

Terminal 
Volts ( F). 

Amps. A. 

Terminal 
Volts (F). 

Amps. J. 








(7) Plot the ascending and descending external Characteristics 
for each speed to the same pair of axes, having voltage ( V) as 
ordinates, and amps. A as abscissae in each case. 

(8) From the above curves deduce, graphically, the total Char¬ 
acteristics on the same curve sheet in the manner described below 
{vide notation, p. 3). 

(9) Plot the H.P. curves on the same sheet. 

(10) Determine the critical resistance for this machine at each 
speed and also the critical current. 

(11) Determine the internal Characteristic of the machine, and 
plot the results, having exciting currents as abscissae and armature 
E.M.F. as ordinates, to the same axes as the above curves. 

(12) Plot the external resistance Characteristic having values 

of V as ordinates and resistance of the external circuit — in 

A 

ohms as abscissae. 

^ Inferences. —Explain the meaning of the curves very carefully. 
How can the effect of alteration of speed for one or more points on 
a Characteristic be corrected for ? Why does the total Character¬ 
istic not droop so much as the external at the higher currents ? 

Graphical Determination of the Total Char¬ 
acteristic from the External One in a 
Series Wound Dynamo. 

Take two co-ordinate axes meeting in the point 0 of which the 
ordinates represent voltage and the abscissae current in amperes. 
Let X be the external Characteristic curve of the series dynamo. 

















ELECTRICAL ENGINEERTNG TESTING 


153 


Take any point Q on the abscissa representing any current 
OQ and set off on the ordinate a line QP to represent OQ 
X + o\^) volts. Join OP, which therefore gives the rate of 
fall of potential down the armature and series coils combined, 
i. e. down the whole machine, or we have tan. POQ = 

Now take any point a' in the curve x and through it draw an 
ordinate aha cutting OP and OQ in h and a respectively; set off 
ah' as shown = whence h' is a point on the total Character¬ 
istic. Kepeat this operation for some ten different points along 



X and finally obtain the total Characteristic y of the machine 
at the particular speed in question, Fig. 59. 

If the curves start at some point e other than the origin 0, 
then Oe is the E.M.F. at that speed due to residual magnetism 
in the field magnets when the external circuit is open, as in 
observation 2 above; Oe will be greater the greater the hardness 
and retentivity of the iron in the magnets. 

If the curves x and y are much separated, it shows that the 
resistance of the machine, e. tan. POQ, is too great, and therefore 
that it cannot be very efficient. Again, the summit G of the 
curv’e x shows for what output {GH x HO Watts) and current 
Oil the armature is magnetically saturated, and the drooping of 
the part Gx gives an idea as to the demagnetizing action of the 











154 


ELEGTBIGAL ENGINEERING TESTING 


armature on the field. It is greater in dynamos in which the 
magnets are relatively less powerful than the armature, and is 
greatest in those machines in which the armature core is more 
nearly saturated than those of the field magnets. 


Graphical Determination of Critical Re¬ 
sistance at a Given Speed for a Series 
Wound Dynamo. 

Let the curve OCx be, say, the total characteristic of the 
machine for a given speed. Then the total circuit resistance 



(iff + 7*0 + 9's) corresponding to any point F can at once be found if 
the resistance line y is known, and which is found as follows— 
Let N = point of intersection of the two axes 10, Fig. 60. 


Then joining ON we have tan. N0M== 


NM 10_ 


Hence iF is a point representing 1 ohm. 

Similarly N' being the point of intersection of the 20 volt 


V 20 

axis and 10 ampere one, will represent ^ = ~ = 2ohms, and so on. 









ELECTRICAL ENGINEERING TESTING 


155 


Thus the whole resistance line yM is obtained. 

Now to obtain the circuit resistance for the point P, draw 
its co-ordinates PQ and PS and join OP. 


Then 


tan 

tan. 10b- 


= 1*375 ohms, 


but 3/^”= 1*375. Hence the resistance of the circuit (7^-f -b 
is given by the point of intersection K of Caojoin of P to 0, and 
the resistance line My. 

If then P approaches the origin 0 just so close that OP 
coincides with OP' and is practically a tangent to the curve {x\ 
then its point of intersection with My gives the cn'itical circuit 
resistance, above which the series dynamo will not excite. 

This point is II and corresponds to 2*60 ohms on the resist¬ 
ance line. Thus for this speed the machine will not work if 
(P-f -f ?*g) is greater than 2*6 ohms. For this instance the 
ordinates represent total E.M.F. of course, but if they were 
terminal volts and Ox the external Characteristic, then MH 
would = the critical external resistance R, 


(59) Determination of the Internal Charac¬ 
teristic or Curve of Magnetization of 
a Series Wound Dynamo. 

This is obtained in precisely the same manner as that in the 
case of the separately excited machine (p. 148), and consequently 
the mode of procedure will not be repeated. The curve should 
be plotted to the same axes as the ordinary Characteristics when 
•the relative positions will form a measure of the armature reaction 
on the field. 


(6o) Determination of the External Charac¬ 
teristic of a Shunt Wound Dynamo. 

Introduction.—In the type of machine under consideration 
only a fraction of the full load current generated is employed 
for the purpose of magnetizing the field magnets, the winding 
of the coils of which consists of a large number of turns of small 




156 


ELECTRICAL ENGINEERING TESTING 


gauge wire, possessing a comparatively large resistance. This 
shunt winding is connected directly across the brushes of the 
dynamo. Shunt dynamos possess a region of approximately 
constant potential, the falling off in this latter being mainly due 
to armature resistance. 

This type of generator is an extremely important one and its 
sphere of application very large. 

The performance and qualities of such a machine can best be 
investigated by means of its Characteristics. Regarding the 
shunt dynamo from a theoretical standpoint, suppose that A^, A, 
and A are the currents flowing through the armature, shunt, 
and external circuit whose resistances are respectively o'g and B, 
then we have Aa = A + A^, since the armature current splits up 
through shunt and external circuits. 

Now since we are dealing with two parallel circuits and one 
common potential difference, we have, that the terminal voltage 
V=AE = Agrl and the parallel resistance of shunt and external 

Rt 

circuit IS -r. -. 

B + Tg 

Hence E = A^ K 

or by substituting for Aa its value A + A^ and reducing we get 


Apparatus. —Shunt dynamo E to be tested, of which EJf 


FM 


represents its field coils in series with 
a high resistance rheostats (Fig. 269); 
voltmeter V ; ammeter A ; main 
variable rheostat R (p. 606); switch /S';* 
tachometer. 

Observations. —(1) Connect up as 
in Fig. 61, and adjust the instru¬ 
ments to zero if necessary. See that 
all lubricating cups in use feed 
properly. 

(2) Start i) up to its normal speed, 
and with S open vary r so as to produce normal excitation, then 
note the value of F. 

(3) With R large, close S and take about ten different values 












ELECTRICAL ENGINEERING TESTING 157 

of current rising by about equal increments from 0 to the 
maximum. Note the voltage V at each and the current A, the 
speed being kept constant at the above value. 

(4) Repeat 2 and 3 for a similar descending set of readings. 

(5) Repeat 2-4 for speeds 20% above and 50% below normal 
respectively. 

(6) Measure, by means of the Wheatstone Bridge, the resist¬ 
ance r, of the shunt coils (warm), and by the “Potential 
Difference” method, the resistance of the armature if very 
low, and tabulate your results as follows—- 

Name ... • Date . . . 


Shunt Dynamo tested: No. . . . '^pe . . . Maker . . . 

Normal; volts . . . Amps. . . . Speed . . . Resistance ra = . . . u= . . . 


Speed 
Revs, per 
Minute. 

Ascending. 

Descending. 

Resistance of 
External Circuit 

mean „ 

— ohms, 

mean A 

Terminal 
Volts V. 

Amps. A. 

Terminal 
Volts (F.) 

Amps. (A). 








(7) Plot the ascending and descending external Characteristics 
for each speed on the same curve sheet, having values of V as 
ordinates and A as abscisses. 

(8) From these deduce, by means of the following graphical 
construction, the total Characteristics on the same curve sheet. 

(9) Plot the H.P. curves to the same axes also. 

(10) Plot the external resistance Characteristic, having values 

V 

of V as ordinates and resistance of the external circuit — ohms 

A 

as abscissae. 

(11) Determine the critical resistance of the machine for each 
speed and also the critical current. 

Inferences. —State clearly all that you can deduce from your 
experimental results, explaining carefully the meaning of the 
shape of the curves. 

Graphical Determination of the Total Char¬ 
acteristic frorh the External Curve in a 
Shunt Wound Dynamo. 

Referring to Fig. 62 : let x be the external Characteristic. 
Take any point Q in the axis of abscissae representing anj; 
















158 


ELECTRICAL ENGINEERING TESTING 


current OQ, and from it set off QR = OQ X volts join OR, 
which therefore gives the fall of .potential in the armature, we 


also have 


RQ 

tan. ROQ —'qq 


r. 


a* 


Again, take any point T on the ordinates representing any 

OT 

voltage OT at the shunt terminals, and from set off TS= — 


/ 



amperes; join OS, which therefore gives the current taken by 
the shunt coils at different voltages, and we have 


tan. 


OT 

SOQ — = Tg 


Now take any point P in x, and draw its co-ordinates Pc and 
Pf of which Pc cuts OS in d. Then ccZ = shunt current = 
and cP = external current A. If now cP is produced to d' so that 
Pd'= cd, then the total armature current Aa = A + Ag = cP + Pd'. 

Now draw an ordinate through d' cutting OR and OQ in h and 
a respectively. Then ah = loss of volts in armature due to total 












ELECTRICAL ENGINEERING TESTING 


159 


current = cd', consequently setting off d'b' = ah we get h' a 
point in the total Characteristic which in a shunt dynamo gives 
the relation between total armature current Aa= {A -4,) and 
total E.M.F. E ~ (F-f- repeating this operation for a 

number of points such as J* on a; we are finally able to draw the 
total Characteristic (y). 

The working part of the curve is that at the top down to the 
sharp bend on the extreme right. 

The critical external resistance R = tan. a for this speed, and 
the machine will only work providing R is greater than tan. a. 

The lower or straight portion of the curves is the unstable 
part. Thus it will be seen that a shunt dynamo would give a 
nearly constant voltage for a given speed and variable external 
resistance, if it were not for the armature resistance. 

(6i) Determination of the Internal Charac¬ 
teristic or Curve of Magnetization of a 
Shunt Wound Dynamo. 

Introduction. —The internal Characteristic of a shunt dynamo 
is similar in shape to the total Characteristic of a series dynamo, 
and is a curve showing the relation between exciting or shunt 
current and the E.M.F. across the brushes for open external 
circuit. Since, however, the shunt current is so small that its 
passage through the armature would not cause any appreciable 
reaction or demagnetization, the shunt may be excited from the 
brushes and the armature allowed to give this small current, 
instead of disconnecting the shunt and separately exciting it 
from an independent source. When the dimensions of the 
magnetic circuit of the machine are known, scales of the axes can 
be marked in air-gap flux density as ordinates, and amp. turns 
per pole as abscissae. 

Apparatus. —Shunt dynamo; variable high resistance r capable 
of carrying the shunt current (p. 599); low reading ammeter (a ); 
switch S ; voltmeter F. 

Observations.—(1) Connect the shunt coils of the dynamo in 
series with r, a and s across the brushes and F also across them, 
and adjust the instruments to zero. 

(2) Take an ascending and also a descending set of readings 


160 


ELECTRICAL ENGINEERING TESTING 


of V and a (at constant speed, say the normal) by varying r, and 
tabulate the results in a convenient manner. 

(3) Plot the internal Characteristic having voltages^ V (x to 
magnetic field flux) as ordinates and shunt currents (a) (x to 
magnetizing force) as abscissae in both ascending and descending 
readings. 

Inferences.—Carefully point out the meaning of the curves so 
obtained. 

(62) Determination of the External Charac¬ 
teristic of a Compound Wound Dynamo. 

Introduction.—The type of machine under consideration is a 
combination of a series and shunt dynamo, so far as the field 
arrangements go, i. e. its field magnets are wound with both 
series and shunt coils. 

The compound dynamo is a self-^'egulating machine for constant 
terminal voltage (at constant speed)^ independent of variations of 
external current. The principle of the self-regulating property 
is as follows :—As the external current increases in the case of a 
shunt dynamo the lost volts due to armature resistance and 
demagnetizing reaction of the armature in the field reduces the 
effective magnetism of the field magnets, but this increase of 
main current causes an increase in the field magnetism due to the 
series coils, which can be made to just counteract the diminution 
due to the lost volts, at otic definite constant speedy thereby 
producing constant voltage at the terminals of the machine. 

There are two possible methods of connecting the shunt coils 
to the machine, and which are shown in Fig. 63 symbolically, 
where I. represents what is called “ Long Shunt ,and II. that 
termed Short Shunt.” are in each case the terminals of 

the dynamo, and as seen in the first case the shunt (fill) is across 
the extreme ends of armature and series coils (se), i. e. across T^T 2 , 
while in the second case it is across the brushes alone. A careful 
study of the dynamo with each method of connection will show 
which is the most desirable arrangement to use in any particular 
case. This, together with the investigation of the performance, 
etc., while working, can best be obtained by means of the 
Characteristics of the dynamo. 


ELECTRICAL ENGINEERING TESTING 


IGl 


Apparatus. —Compound wound dynamo D to bo tested ; switch 
S) voltmeter V ^ ammeter A rheostat R (p. 606) j tachometer. 

Long Shunt. 

Observations. —(1) Connect up as in Fig. 63 (I.), and adjust 
the instruments to zero if necessary. See that all lubricating 
cups feed slowly and properly. 

(2) Start D up to its. normal speed, and when excited note 
the reading on F. 

(3) With R large close and take about ten different values 
of current A rising by about = increments from 0 to the 


sh sh 



maximum allowable. Note the voltage V at each, the speed 
being constant at the above normal value. 

(4) Repeat 2 and 3 for a similar descending set of ob¬ 
servations. 

(5) Repeat 2-4 for speeds 20% above and 50% under 
normal respectively for constant excitation in each case in the 
shunt coils. 

(6) Measure by means of a Wheatstone Bridge the resistance 
Vsh of the shunt, and by^ the “ Potential Difference ” method 
(p. 84) that of the armature and series coils 

(7) Disconnect either coil in turn and repeat the above 
obs. 2-6 with the remaining coil, and so obtain the external 
Characteristic of the dynamo for each winding separately. 

M 















162 


ELEGTRICAL ENGINEERING TESTING 


Note.—In doing this with sh connected alone, obtain the same 
initial voltage V as when mnning compound. 

Tabulate all your results as follows— 


Name . . . Date . . . 

Compound Dynamo tested : No. . . . Type . . , Makei . . 

Normal Volts= . . . Amps. = . . . Speed = . .. Resistances ra= ... rse = . . . rsh= .. 


Winding 

used. 

Speed 
Revs, 
per min. 

Ascending. 

Descending. 

• 

Resistance of 
External Circuii 

mean F , 

— on ms. 

mean A 

Terminal 
Volts (F). 

Amps. (A). 

Tei minal 
Volts ( V). 

Amps. (A). 









(8) Plot the external Gliaracterisiics for each speed, and both 
ascending and descending observations, having voltages V as 
ordinates and currents A as abscissie. 

(9) From these deduce graphically, as described below, the 
total Characteristics^ drawing them on the same sheet of paper. 
(For notation, see p. 3.) 

(10) Plot the H.P. curves to the same axes also. 

(11) Plot the external resistance Characteristic having values 

of V as ordinates and resistance of the external circuit — in ohms 

A 

as abscissae. 

Inferences. —State carefully all that can be inferred from your 
results, and explain the meaning of the various curves. 


Graphical Determination of the Total Char¬ 
acteristic from the External one, in a 
Compound Wound Dynamo. 

Long Shunt. 

Referring to Fig. 64, let x be the external Characteristic. 
Take any point Q in the abscissae representing any external 
current OQ, and set off from Q on the ordinates QR=OQ 
('f’a + ^se)j whence tan. ROQ = (ra + r^g). Again, taking any point 
2' representing any voltage OT at the shunt terminals, set oil 

02 ^ 

from T a line TS = -whence tan. SOQ = rgh. 

'i'sh 





















ELECTRICAL ENGINEERING TESTING 


163 


Now take any point P in aj and draw its co-ordinates PC and 
Pn, then the intercept Cd between OT and OS = shunt current 
at this voltage Pn. As in the case of the pure shunt machine 
make Pd' = Cd, and draw an ordinate d'a through d' cutting OB 
and OQ in h and a. 

Then making d'h' = ah, we get h' to be a point on the total 
Characteristic (y). Repeating this process for several points, such 
as P along {x), we finally get the curve y showing the relation 
between total armature current which = and total E.M.F. 

E, which = VI- A where in Fig. 64 On = A,na, = A^ = cd, 

.*. Oa = On ^-na = A+ A^ 
ah = Oa (r„ + (?•„ + 



{63) Determination of the External Charac¬ 
teristic of a Compound Wound Dynamo. 

Short Shunt. 

As the mode of procedure is precisely the same as in the case 
of the corresponding long shunt test, this latter will not bo 













1G4 


ELEGTRIOAL ENGINEERING TESTING 


repeated here, and must be referred to both for apparatus, which 
is the same in both cases, and for the diagram of connections which 
is shown in Fig. 63 (II.). 

Graphical Determination of the Total Char¬ 
acteristic from the Internal one in a 
Compound Wound Dynamo. 

Short Shunt. 


Referring to Fig. 65, let x be the external Characteristic. As 



Fig. 65. 

in the last case take any current OQ and set off QJF= OQ x rgg 
and QR = OQ x Va and join OR, OW. 

Obtain OS as before, and let F = any point on the curve (ic). 
From P draw Fc = GR, and from c draw cd' = cd; lastly, make 
d'b' = ab and b' is a point on the total Characteristic y, thus 2 / 
is easily obtained. By reference to Fig. 65 it will be seen that the 
total E.M.F. F++ while.the total current in the 
armature Aa = A + or in this Fig.— 

ttb = FG + GIIA cib—V Axg^ + A^r^ 

Oa = OG + Ga =A + AgJ^. 


and 











ELECTRICAL ENGINEERING TESTING 166 

(64) Determination of the Separate Field 
Magnet Windings for Truly Compound¬ 
ing a Dynamo. 

Introduction. —This is a practical method of experimentally 
determining the number of amp.-turns of excitation to be 
supplied by both the series and the shunt coils in a compound 
machine without having to calculate them in the ordinary way 
from data pertaining to the magnetic circuit. The method forms 
an exceedingly instructive one for use in a laboratory, in illus¬ 
trating theoretical principles, but it is essentially a works 
method, and has the advantages that the amp.-turns are 
determined under full load working conditions, i. e. when the 
armature is exerting its maximum demagnetizing influence on 
the field, and furthermore that since the whole machine is already 
con.structed except for these coils, any errors in the lengths and 
sections of the various parts of the magnetic circuit and in their 
estimated permeabilities are nullified. 

The conditions for automatic self-regulation by compound 
winding are, firstly, that with the external circuit open, i. e. no 
current through the series coils, the shunt winding must alone 
produce, at the given speed, the full specified voltage of the 
machine. Secondly, that on full load the series or winding must 
supply such an extra amount of magnetization, due to the full¬ 
load current in its coils, as will main¬ 
tain the same specified voltage of the 
dynamo. There are obviously two 
modes of procedure differing slightly 
from one another and depending on 
circumstances at hand, namely, (1) to 
compound a machine given its carcase 
ready built up and the armature ready 
wound; (2) to compound a shunt 

machine by finding the necessary series 
turns to be added. Consider the former 
conditions first. 

‘ Apparatus. — In addition to the 
machine in question—a voltmeter V] 
switches S and ; ammeters A and 
a ; rheostats R (p. 606) and r (p. 599); 
source of E.M.F. {B) for excitation, 
either battery or other dynamo, etc. 



NA) — 

Fig. 66. 



















1G6 


ELECTRICAL ENGINEERING TESTING 


Determinations.—(1) Take a suitable field frame of the type 
of machine specified, i. e. one that previous experience shows to 
be of sufficient size for the work, also a suitable ready-w’ound 
armature, wound with a suitable number of turns of a gauge 
sufficient to take the specified full-load current of the machine 
at an orthodox current density in the coils. 


(2) Wind the field magnet limbs uniformly with a few tem¬ 
porary turns F (known) of thick wire or lead, and connect up 
as in Fig. 66, adjusting the instruments to zero. 

(3) Run the armature at the specified speed {n) revolutions 
per minute, and with S open close S^, adjusting the exciting 
current {a) by means of r until V reads the specified voltage. 
Then aF = amp.-turns to be supplied by the shunt coils alone. 

(4) Now close S, and obtain the full-load current A, of 
specification, at the same speed [n) above ] again adjust r so that 
V again reads the same specified voltage as before. If a^ now 
= the exciting,current, then a^F = amp.-turns to be supplied 
by (series -P shunt) together; hence {a^ ~ a)F = amp.-turns to 
be supplied by series coils alone at that voltage V and speed (ii) 

, - . (^1 “ 
or number or series turns = —-—-. 


Such a gauge of wire is then chosen for the shunt coils, that 
with a shunt current of, say, 2% of A (the main one) the shunt 
V V 

resistance = 2 ^ = 50^ ohms and the amp.-turns = aF. 

Too 


If we are dealing with case 2, in which it is desired to convert 
a shunt machine into a compound one, it should be remembered 
that such an alteration is only possible when the field magnets 
are not magnetically saturated. 

The mode of procedure is then practically the same as before, 
and is as follows— 

Disconnect the shunt and separately excite it, so as to reproduce 
the same terminal voltage (at full load) as the shunt (by itself) 
gave with the armature on open circuit. 

If then the total number of shunt coil turns = T and the 
shunt current rose from the original value (a) to the new value 
(^i) in order to keep up the same voltage at full load as on open 
circuit, then (a^ - a)2^ = amp.-turns to be supplied by series coils; 





ELECTRICAL ENGINEERING TESTING 


1G7 


and if the full-load armature current is A, then number of series 
turns = ^ the speed being constant all the time. 

It will be obvious that the method serves to determine the 
windings necessary to over compound, the exciting current at full 
load being raised until the excess over the normal voltage = the 
drop of volts in the mains at that current. 


(65) Determination of the Speed and E.M.F. 
at which a Dynamo truly Compounds. 

Introduction. —The present test of course relates to a com¬ 
pound wound dynamo which is already built and finished. It 
has been previously mentioned that a given machine can only 
compound truly and exactly to give constant voltage for wide 
variations of external current, at one particular definite speed. 
This is owing to the different alterations which a definite variation 
of speed produces on the series and shunt Characteristics of the 
machine. Thus, if after a machine is built and completely finished 
it is found that the compounding is not quite correct at the speed 
used in the calculations, which could easily be the case, then 
the speed would form a means of final adjustment and the mode 
of procedure would be as follows— 

{a) With the machine connected so as to self-excite in the 
usual way, place an external circuit consisting of an ammeter 
A, rheostat R, and switch S in series with the machine terminals 
and a voltmeter V across them. 

(6) Run the dynamo at the speed employed in the design, and 
take first the open-circuit volts Y and then that at some 4 or 5 
loads between 0 and the maximum. 

(c) If on plotting these observations the Characteristic thus 
formed droops, the series coils are too weak in their effect and 
the speed should be raised a little, another set of readings being 
taken. 

In this way a speed will be found by trial such that the curve 
is a horizontal straight line or nearly straight between 0 and 
full load. The speed and volts at the terminals now are the 
values required for exact compounding. 



168 


elFjGtbical engineering testing 


(66) Variation of the E.M.F. of an Alternator 
with Speed at Constant Excitation. 

Introduction. —The present test is an important one, as 
showing not merely the effect of alteration of speed on the 
E.M.F. developed by an alternator for constant exciting 
current, but also whether the demagnetizing effect of the 
armature on the field, due to eddy currents generated in it, is 
producing a perceptible effect, and if so the speed at which this 
effect begins to assert itself most forcibly. The eddy current 
loss varies as the square of the speed for constant magnetic 
field, so that the results of the test will give a measure of 
the adequacy of the lamination of the iron parts of the 
machine. 



Apparatus. —Alternator D to be tested ; alternating current 
voltmeter V; speed indicator and means of driving I) at any 
suitable speed. For the exciting circuit, in addition to the 
field coils F of the alternator—a variable rheostat B (p. 599) • 
switch S; ammeter A ; exciting E.M.F. e consisting either of the 
requisite E.M.F. from a secondary battery or auxiliary direct 
current supply, etc. 

N.B.—Since in this test the speed is variable, and con¬ 
sequently also the periodicity of the current, the voltmeter V 
should bo either a “hot wire” or “electrostatic” instrument, 
these being the only two types of meters which are unaffected 
by the variation of periodicity. 














ELECTRICAL ENGINEERING TESTING 


1G9 


Observation. —(1) Connect up as in Fig. 67, and adjust the 
pointers of the instruments to zero. See that all lubricating 
cups in use feed properly and slowly, and then start the 
alternator. 

(2) Close S, and adjust the exciting current on A to the 
normal for the machine by altering R, and keep it constant; then 
adjust the speed so as to obtain the lowest readable voltage on V ; 
note simultaneously the speed and voltage V. 

(3) Repeat 2 for about ten different speeds, rising by about 
equal increments from the above value to the highest allowable 
(a) at the same constant normal excitation, (6) at a constant 
value 50% less. 

(4) Repeat 2 and 3 for the same constant exciting currents 
with constant full-load armature current in each case respectively, 
and tabulate your results as follows— 


Name . . . 


Date . . 

Alternator tested : No. . . . 

Type . . . 

Maker 

Normal Full-Load Volts = . . . 
Normal Exciting Current = . . . 

Amperes = , ■ « 

Speed• 


Speed 

Revs, per min. 

Voltage (terminal) 

V 

Exciting Current 

A Ami'S. 





(5) Plot curves having E M.F. {{. e. terminal voltage) as 
ordinates and speed as abscissae for each excitation both on no- 
load and full-load to the same pair of axes. 

Inferences.— Carefully state all that can be inferred from the 
results of the above test, and point out their bearing on the 
design of alternators. 


(67) Variation of the E.M.F. of an Alternator 
with Excitation at Constant Speed 
(Magnetization or Open-Circuit Charac¬ 
teristic). 

Introduction.—This test is an important one, in that it shows 
the degree of approximate magnetic saturation of the field 
magnets at any excitation, and in conjunction with the short- 








170 


ELECTRICAL ENGINEERING TESTING 


circuit characteristic will give a predetermination of the regu¬ 
lation of an alternator for terminal E.M.F., on load. If the 
magnetization of its field magnets is carried too close to the 
point of saturation, then manifestly it will require large 
variations of exciting current to produce comparatively small 
variations in the E.M.F. of the machine. This, it need hardly 
be pointed out, is undesirable, and would cause a most 
insensitive form of regulation. Again, it will be apparent that, 
if no demagnetizing action of the armature on the field is going 
on, the exciting (i. e. magnetizing) current will represent the 
magnetizing force and form a measure of it. This in turn will 
create a certain magnetic field and magnetization, and in this 
field the armature rotates, or as in the inductor form of alternator 
this field is made to change from zero to full and back to zero, 
and in so doing cuts the stationary armature conductors. But 
the E.M.F. generated is cc rate of change of this field, and at 
constant speed to the field strength itself. Hence the E.M.F. is 
a measure of the magnetic field or induction excited, providing 
there is no armature reaction. Thus the present test will give 
us approximately the curve of magnetization of the alternator 
field magnets, but it must be carefully remembered that this is 
only approximate and provided there is no armature reaction on 
the field due to eddy currents in the armature, which should not 
occur when the machine is running on no load. 

Apparatus. —Precisely the same as that used in the preceding 
test. 

Observation. —(1) Connect up exactly as indicated in Fig. 67, 
and adjust the instruments to zero. See that all lubricating 
cups feed properly and slowly, and then start the alternator. 

(2) Adjust the speed of the alternator to the normal value for 
that machine, and keep it constant. Close S and adjust the 
exciting current (by varying A), so as to obtain the lowest 
readable voltage on V. Note this simultaneously with the 
exciting current and speed. 

(3) Repeat 2 for about ten different exciting currents, rising 
from the preceding amount to the maximum allowable, the speed 
being constant at the above value throughout. 

(4) Take a similar descending set of observations as in 3. 


ELECmiGAL ENGINEERING TESTING 


111 


(5) Repeat 2 and 3 for (constant) speeds 50% below, and if 
possible 20% above normal^ and tabulate your results as 
follows— 


Name . . . Date . . . 


Alternator tested: No. . . . Type . . . Maker . . . 

Normal Voltage = . . . Current . . . Speed . . . 

Normal Exciting Current = . . . 


Speed 

Revs, per min. 

Exciting Current 

A Amps. 

Terminal Voltage V. 

Ascending 

Descending 

Ascending 

Descending 







(6) Plot curves for each speed to the same axes having volts 
as ordinates and exciting currents as abscissse, both for 
ascending and descending readings. 

Inferences, —State very clearly all that can be inferred from 
your results, and point out their bearing on the design of 
alternators. 


(68) Magnetization Curve of an Alternator 
on Full Load—Non-Inductive and In¬ 
ductive. 

Introduction. —While the “ no-load magnetization ” curve or 
“open circuit” Characteristic of both an alternator and D.C. 
dynamo take the same general form, the curve which, as we have 
seen, relates terminal voltage of armature with excitation current 
is different on full load, and in the case of an alternator depends 
on the nature of the external load, i. e. whether inductive or non- 
inductive. This difference is due to the loss of terminal voltage 
on load caused by the ohmic resistance, reactance, and reaction 
of the armature when delivering current; though only resistance 
causes loss of power. Armature reaction is responsible for dis¬ 
torting and either strengthening or weakening the main magnetic 
field in the air gap, and is due to armature current and depends 
on the power factor of the external circuit, while armature 
reactance, due to the self-induction of the armature conductors, 














172 


ELECTRICAL ENGINEERING TESTING 


causes the current to lag behind the induced voltage, though in 
phase with the terminal voltage, and is unaffected by the power 
factor of the external circuit. The magnetization curve on full 
load, when compared with that on no load, affords valuable 
information needed for the design of a field regulator for the 
alternator to give any particular degree of sensitiveness. 

Apparatus. —Precisely that given for Test No. 70. 

Observations.—(1) Connect up as in Fig. 69, levelling and 
adjusting such instruments as need it to zero. See that the 
lubricating arrangements are working properly on starting up 
the motor alternator, which should be the same as that used in 
obtaining the “open circuit” Characteristic of Test No. 67. 

(2) With a variable non-inductive resistance R connected for 
absorbing the output, and with R and r full in, adjust the speed 
of the alternator to its normal value and maintain this constant 
throughout by field regulation on the driving motor. Now 
close S-^ and S and reduce R until the alternator gives full-load 
current on A, then note the readings of F, and (a) at constant 
normal speed for this and a series of exciting currents (a) 
decreasing by about equal amounts to the lowest possible, the 
armature current A being kept constant by reducing R. 

(3) With a variable ind,uctive resistance for R —composed 
either of an adjustable choking coil in series or parallel with 
adjustable non-inductive resistance, or of a synchronous motor, 
the excitation and loading of which can be varied {vide p. 305), 
and the addition of a wattmeter for measuring the power absorbed 
in R, Fig. 69. Repeat obs. 2 for constant power factors cos 
of, say, 0*9, 0-8, etc., leading and lagging if possible, and tabulate 
all your results as follows— 


Alternator tested : No. = . . . Type . . Maker . . . 

Full load: Volts = . . . Amp^;. = . . . Speed = . . , 


Nature of load 

Ind. or Non-Ind. 

Speed. 

Armature 

Amps. 

A. 

Exciting 

Amps. 

a. 

Volts 

V. 

Walts 

W. 

Power F.ictor 

JK 

cos </) = - 

A V 

. 








(4) Plot (from obs. 2 and 3) curves of full-load magnetization 
to the same axes, having volts V as ordinates with exciting 
currents {a) as abscissae. 

















ELECTRICAL ENGINEERING TESTING 


173 


(5) For comparison replot the “no-load” magnetization curve 
obtained in Test No. 67 for the same alternator on the above 
curve sheet. 

Inferences.—What can you deduce from the results of the 
above test, and how can the range of field regulating resistance 
be obtained for maintaining constant voltage between 0 and full 
load on any particular power factor of circuit ? What excitation 
is necessary to send full-load short-circuit current through the 
armature at normal speed ? 


(69) Determination of the Short-Circuit 
Characteristic of an Alternator. 

Introduction. —This characteristic, which is really the magnet¬ 
ization curve of the alternator on short circuit, differs from that 
obtained in Experiment 70, in that in conjunction with the 
“open ” circuit characteristic of the machine, p. 169, it forms a 
means of predetermining the “voltage drop,” i.e. the regulation 
of any alternator at different external loads and power factors. 
In this way large alternators may be tested while giving full- 
' load current although requiring very little power to drive them. 

Apparatus. —Precisely that detailed for Experiment 70, the 
resistance R being capable of variation and of being short 
circuited. 

Observations. —(1) Connect up precisely as in Fig. 69, and 
adjust the pointers of such instruments as require it, to zero. 
See that all lubricators in use feed slowly and properly. 

(2) Start the alternator up to normal speed and close noting 
the readings of F and A (if any) with R cut out to short circuit. 

(3) With (r) full in, close (s), and adjust the exciting current 
(a) (by varying r) to some small value that will cause A to read 
about j^th of the full load current of the alternator. Now read 
all the instruments. Next open E and again take readings, the 
speed being kept constant at the normal value throughout. 

(4) Close S again, and increase A by about another by 

suitably increasing the exciting current {a). Then note the 


174 


ELECTRICAL ENGINEERING TESTING 


readings of all the instruments. Next open S and again take 
readings, the speed being constant at normal value thioughout. 

(5) Repeat (4) for a series of values of A up to 20% above full 

load value. 

(G) To determine the effect of speed variation on the short- 
circuit current, run the alternator up to max. safe speed and 
raise its excitation until full load, or 20% over full load, current 
is obtained. Note this current, the speed, excitation and 
voltage and also for some ten different speeds, decreasing by 
about equal amounts to the lowest convenient at constant 
excitation, and tabulate in the following manner. 

jjote.—To avoid the risk of damaging the armature of the 
machine, especially if a large one, the series resistance R should 
all be cut into circuit before closing and again before opening S, 
and should be cut out to short-circuit just before taking instrument 
readings. 


Alternator: No. . . . Typo . . . Maker . . . 

Full Load Output. Amps. Volts • . . revs, per min. 

„ ,, Excitation = . . . Amps. Frequency = . . . .^ per see. 

Resistance (warm) ol Aiiuatiire + Ammeter and Leads R = . . . ohms. 
Percentage Allowance K (if any) = . . . 

Total Equivalent Resistance Rt = {R + KR)=. . . ohms. 


Speed 

(con¬ 

stant). 

Terminal Armature Volts. 

Short 

Circuit 

Armature 

Current 

A. 

Exciting 

Current 

a. 

Effective 
Volts for 
Ohmic 
Resist. 
ARt. 

Idle or 
Inductive 
Voltage Drop 
^/E^-{ARt)'^ 
=-Ei. 

Short 

Circuit 

Ea. 

Open 

Circuit 

E. 









(7) Plot the “ short-circuit characteristic ” having values of (d) 
as ordinates and (a) as abscissae, and also the curve having A as 
ordinates with speed as abscissie. 

Inferences.—State all that can be deduced from the results of 
the test. 

Determination of the Voltage Drop ” of an 
Alternator for any given load and circuit 
Power Factor. 

From the results of the above test and with the aid of a simple 
graphical construction, it is easy to find approximately the voltage 
drop corresponding to any load current and power factor. Thus— 




















ELECTRICAL ENGINEERING TESTING 


175 


From any point 0 in any straight line JTFset off OE, to a con¬ 
venient scale, to represent any desired ‘‘open circuit” voltage of 
the alternator and at such an angle $ to the current line XF, that 
the power factor (cos. 6) of the circuit has the desired value for 
the particular load current ( = short circuit current A) assumed. 





With centre 0 and radius O^draw the semi-circle E^YE^. Next 
set off along XY^ the line OG = AB^ the armature drop and 
allowance (/i) for eddy current loss, corresponding to the same 
main current A. 

Note. —In small, slow speed machines, the power lost due to 
eddy currents in armature core and pole pieces is small, and the 
effect of this on the voltage drop is also small compared with the 
term AR and can be neglected. When, however, the effect is too 






176 


ELECTRICAL ENGINEERING TESTING 


large to neglect as in large machines the term NR can be increased 
by a certain percentage {K) depending on the form of alternator 
and the speed, (as dictated by experience with this type of 
machine or by experiment) to allow for the eddy current effect. 
Since the term ARx is small compared with E, the error intro¬ 
duced in the value of the inductive drop Ei, through estimating 
the percentage increase of AR wrongly, is very small. 

From G draw CR perpendicular to XT and = the short-circuit 
inductive or idle voltage drop Ej due to the self-induction of the 
armature and loss of voltage due to armature reaction. Join OR 
which therefore = the open circuit voltage E, which is necessary 
to overcome the self-induction and resistance of the armature and 
leads, for the short-circuit current A assumed. OCR is therefore 
the right-angled triangle of E.M.F.’s on short circuit and the angle 
COR = the angle of lag between current and voltage on short 
circuit. Now with centre R and radius OE draw the semi-circle 
Fj FF 4 . Then OV = terminal voltage of the machine and VE the 
voltage drop for the main circuit current A and circuit power 
factor cos, 6. OV and VE can similarly at once be found for any 
other value of power factor with the same main current A from 
the same diagram. A new diagram must, however, be con¬ 
structed in a similar manner for a different current A in order 
to obtain the terminal volts such as OF and drop such as VE for 
this new current. Some interesting cases are now obvious, 
namely— 

For main current lagging behind the voltage, i. e. self-induction 
predominating in the main circuit. 

( 1 ) For main circuit power factors corresponding to an angle 

= OOA, when OE takes the position OA^, in line with RO, the 
terminal voltage OFj = minimum and the inductive drop 
V^E^ = maximum. 

(2) For a non-inductive main circuit, 6 = 0 and the terminal 
voltage OY is in phase, and coincides in sense, with the main 
current, but will differ from the resultant voltage OR by a little 
more than 90°. 

For main current leading in front of voltage, i. e. capacity pre¬ 
dominating in the main circuit. 

(3) For the negative angle 6^, terminal voltage OFg = open 
circuit voltage OE^ and there is no inductive drop. 


ELECTRICAL EmiNEERTNG TESTING 


177 


(4) For greater “leads,” tlie terminal voltage is greater than 
tlie open circuit voltage, e.g. for a —angle 0^ the terminal voltage 
= 0V^ and open circuit voltage OE^, maximum values of these 
quantities being reached at 0 ^ 4 'and where the maximum 
inductive drop = ^ 474 . 

(70) Determination of the External Charac¬ 
teristic of an Alternating Current 
Generator. 

Introduction. —This test is for the purpose of experimentally 
determining, for different speeds and excitations, the relation 



Fio. 69. 


between external current and the terminal voltage producing it. 
By means of the Characteristic curve, as it is called, so obtained 
by plotting the observations, it can be seen at a glance whether 
the alternator possesses any series defects. Its performance 
when running on some particular circuit at some particular 
speed and excitation is also easily discernible therefrom. 

Apparatus. —Alternator D to be tested ; switch S ; alternating 
ammeter A ; and voltmeter V ; non-inductive resistance R, which 
should preferably consist of either a bank of glow-lamps (p. 
598), carbon (p. 597) or water rheostat, all of which are non- 
inductive; in circuit with the exciting coils F —a rheostat r 
(p. 599 ) ; switch ammeter {a ); and source of direct current 
e from secondary cells or otherwise; speed indicator. Watt¬ 
meter W (not shown) for measuring the true watts absorbed 
in R when this is inductive. 

Observations. —(1) Connect up as in Fig. 69, and adjust the 
pointers of all the instruments to zero, if necessary. 

N 














178 


ELECTRICAL ENGINEERING TESTING 


( 2 ) Start the alternator up, seeing that all lubricating arrange¬ 
ments in use feed properly. Close adjusting both the speed 
and excitation to the normal for that machine. 

(3) With this excHation and sjjeed constant^ take, by varying 
R, a set of readings for about ten different values of external 
current between 0 and the maximum permissible, differing by 
about equal amounts, and note the simultaneous readings of V 
and A. 

(4) Repeat (3) with the same normal excitation, hut for constant 
speed 50% below normal. 

(5) Repeat (3) and (4) for a different excitation, say, 50% 
lower than normal, and tabulate all results in the accompanying 
form. 

( 6 ) Measure the resistance of the armature, while warm, by the 
“ Potential Difference ” method (see p. 84), or by the ammeter 
voltmeter method, p. 86 . 

Name . . . Date . . . 

Alternator tested: No. . . . Tj’pe . . . Maker . . . Periods per Kevol. K= . . . 

Norn^al Volts . . . Amps. . . . Sj eed . . . 37 = 277 x Frequency. 

Armature : Self-induction Z = . . . Flenry : Ohmic Resistance Warm . . . 


Speed 
Revs, per 
min. 

Exciting 
Current 
(a) Amps. 

Volts. 

V. 

Amps. 

A. 

Watts 

JF. 

App. 
Watts 
Ax V. 

Power 
Factor 
cos (f> = 
IF 

af' 

Induction. 

Resistance. 

Lp. 

E.M.F. 

L 2 )A. 








1 


(7) Plot the external Characteristic curves for each speed and 
excitation having terminal volts V as ordinates and current A 
through armature and external circuit as abscissae, in each case 
(see p. 3 on curve plotting) using the same pair of axes and curve 
sheet. 

(8) Deduce the total Characteristic curves of the machine from 
the “external" ones in 7 above by means of the graphical method 
given below. 

Inferences. —Very carefully state all that you can infer from 
your experimental results, and show in what way the above 
curves indicate the performance of the alternator, and give an 
idea as to the goodness of the efficiency. 





























ELECTRICAL ENGINEERING TESTING 


170 


Graphical Determination of the Total Char¬ 
acteristic of an Alternator from the 
External Characteristic. 

The total Characteristic of an electrical generator is the curve 
showing the relation between the total E.M.F. in volts generated 
by the machine and the total armature current in amperes. 

Analytical Treatment. 

Heferring now to the preceding diagram (Fig. 69), let Z- self 
induction of the armature in henries— 

r^ = ohmic resistance of the armature in ohms. 

A = ohmic resistance of the external circuit, assumed to be 
non-inductive. 

= angular velocity of the alternating current = 27r7i. 
n = frequency or periodicity in — per sec. 

If then a current A flows in the external circuit at a terminal 
voltage F and A = total E.M.F. of the generator, 

E 

then ^ = V LY + {R + r„)2 but r=AR-, 

hence E=^J LyA^ + {R + = ^LYA^ + {Ar^ \ Yf. 

Consequently we see that the total E.M.F. in volts generated 
by the alternator is represented by the hypothenuse of a right- 
angled triangle, of which the other two sides represent ZpA—the 
inductive E.M.F. necessary to overcome that of self-induction, 
and {Ar^ -f V )—the effective E.M.F. necessary to send the current 
A through the ohmic resistance {R -f I'a). 

If A^= number of periods per revolution of the armature, 

KN . o 

then = (w), the frequency in oo per sec.; whence p — 

where xY= number of revolutions per minute the armature is 
making. 

Having thus obtained an expression for the total E.M.F. 
of the machine analytically, we will now proceed to deduce 
it graphically from our external Characteristic. 







180 


ELEGTBICAL ENGINEERING TESTING 


Grapdical Treatment. 

Let represent the external Characteristic plotted to the 
rectangular axes OT and OS, of which the former represents 
voltage and the latter current, respectively. (See Fig. 70.) 

From the point 0 or origin draw the straight line OQ such that 
tan. QOS — i'a, and also draw the straight line OP such that tan. 
POS = Lj) for the particular speed at which xij is taken. 



Fig. 70. 

Take any point N on the external Characteristic xy and 
through it draw an ordinate MNB cutting OP, OQ and OS in the 
points G, 11 and B, respectively. 

Set off ND = Bll and BG — BG and join DC, which therefore 
= E. Lastly, make BE = CD. 

Then ^ is a point on the total Characteristic required. If this 
construction is repeated for several more points, such as N on xy, 
finally obtain a series of points such as E, and on drawing a 
mean curve through them all we obtain the required total 
Characteristic xz. In this construction it will be noticed that—• 

^iV^=the terminal voltage (V). 









ELECTEIGAL ENGINEERING TESTING 


181 


EG = EG =tlie inductive E.M.F. {L 2 )A). 

Bll = ND — iliiQ effective E.M.F. {Ar^). 

DBG is consequently a right-angled triangle, and 

hence DG = E= V7zP)H(I^^7+T)2. 

Probably a better mode of procedure and one not requiring the 
use of a protractor in obtaining the correct angle at which to set 
off OP and OQ is to first take any point N on xy, obtain the 
ordinate MNB and set off BG = the current OB x Lp, also 
B[I=OB XTai then joining these points II and to 0 we 
get the required lines OIIQ and OGP respectively, after which 
proceed as above. 

(71) Determination of the External Charac¬ 
teristic of an Alternator for different 
circuit Power Factors. 

Introduction. —The circuits which have to be supplied from 
alternators, in ordinary commercial work, are invariably inductive, 
possessing either self-induction or capacity or both. As the 
inductiveness of the circuit affects the terminal voltage of an 
alternator,—self-induction causing the characteristic to fall and 
capacity causing it to rise, it is important to be able to determine 
the characteristic for any power factor of the circuit, whether the 
current lags or leads with respect to the voltage. 

Lagging currents can easily be obtained with any power factor 
by suitable combinations of self-induction and ohmic resistance, 
as for example by carbon rheostats and continuously variable 
choking coils or dimmers. 

Leading currents can be obtained by inserting along with, say 
carbon rheostats (1) a number of condensers possessing consider¬ 
able capacity, (2) a long length of concentric cable, (3) a synchronous 
a.c. motor with fields excited so that the back A. J/.A. of the motor 
exceeds the voltage of the a ternator. This is by far the most 
convenient method, for any load can easily be taken from the 
alternator by either braking the motor pulley or making it drive 
a d.c. dynamo at various loads. It can be shown that over-excited 
synchronous motors cause a leading current to flow in the circuit 
supplying them, and hence produce the same effect as 1 and 2 
above. By adjusting the excitation of such a motor any lead can 
be obtained within limits. 




182 


ELECTBIGAL ENGINEERING TESTING 


Where the only means available for obtaining lagging power 
factors is by varying combinations of self-induction and ohmic 
resistance, or for obtaining leading P.F.s is by varying com¬ 
binations of capacity and ohmic resistance, it is a very tedious 
process to try and take a series of readings of output current 
with terminal voltage at constant I\F. This is obvious since it 
entails varying the inductive and non-inductive sections of the 
load circuit so that they always bear the same ratio to one 
another, as only this will keep cos ^ constant in value, since the 

„ ohmic resistance ,, i i 

F.h. or cos <h — -^^-. in other words, trial com- 

impedance 

binations would have to be repeatedly made and the value of 
W 

-TTr worked out for each, to see if it had the desired value for 
A y 


cos </). For this reason the following method is better, and is 
applicable to any appliance capable of giving an electrical 
output which has to be absorbed in an inductive circuit. Since 
the output obviously comprises (amps A X volts F), both of 
which are variable simultaneously between 0 and full load, and 
V 

A = . -^-or F = ^ X impedance, it follows that A will be 

impedance 

constant if V and the impedance vary in the same proportion, 
while F will be constant if A and the impedance vary in inverse 
proportion. Thus if the inductive portion of the load circuit 
is adjusted to max. value, and the non-inductive portion in series 
or parallel with it (whichever is found most suitable) is varied, 
it is possible to obtain different values of A corresponding to 
different values of cos ^ at constant F, or different values of F 
corresponding to different values of cos at constant A, and a 
curve can be plotted between cos ^ as ordinates and the 
variable as abscissas. If, now, the inductive portion is made less 
inductive and the non-inductive part again varied, a new value 
will be obtained for the constant and a new series of values of 
cos <jE) and the variable obtained, which will give a second curve 
with cos ^ as ordinate and the variable as abscissae. 

Repeating the process for some five or six degrees of iriductive- 
ness, giving therefore as many values of the constant and 
corresponding curves between cos cj) and the variable, we can 
utilize these auxiliary curves to obtain the same number of 





ELECTRICAL ENGI^^EERING TESTING 


183 


curves between V and A each at any desired constant power factor 
(cos <^) within the limits produced by the range in degree of 
inductiveness used. As an example—Suppose A is the constant 
and V the variable with cos <^, and that six auxiliary curves, each 
obtained for, and marked with, the corresponding constant, are 
plotted on the same sheet. Now to obtain the curve between 
V and A at, say, 0’9 power factor : note the six points or 
voltages on the volt scale cut by the ordinates through the six 
points of intersection between the single horizontal line through 
the 0*9 mark on the cos ^ scale and the six curves. Then the 
six voltage values so found, plotted against the six constant 
values of A marked on the respective curves, will give the 
desired relation between A and F at a constant power factor 
cos <f) = 0‘9. 

Similarly, a curve between A and V could be drawn for 
cos (fi = 0*8, 0 7, 0‘6, etc. 

Apparatus. —The same as that detailed for Test No. 70, 
except that the load resistance R must now comprise a variable 
self-induction and variable non-inductive resistance, and that a 
wattmeter W must be inserted so as to give the true watts 
absorbed in the whole circuit. 

Observations. —(1) Connect up as in Fig. 69 with the slight 
modifications just mentioned. Level and adjust the pointers of 
any instruments needing it to zero, and see that the lubricating 
arrangements are in operation on starting up the machine. 

(2) With S closed and with both excitation and sp)eed adjusted 
to the normal value and maintained constant, vary the pro¬ 
portions of heavy self-induction and large ohmic resistance 
in the main circuit, so as to obtain some six difi’erent volt¬ 
meter and corresponding wattmeter readings for each of five 
dififerent but constant alternator currents A, differing by about 
equal amounts between 0 and full load, the same value of main 
current to be obtained for each of the six readings of a set. 

(3) Plot five curves one for each of the five constant values of 
main current, each having the terming,! voltage of the alternator 
as abscissae and power factor (obtained by indicator or wattmeter 
and volt-amperes) as ordinates. Mark on each curve the constant 
main current at which it was obtained. 

(4) Repeat obs. 2 and 3 so as to obtain a similar set of five 
curves for leading currents of the same magnitude as previously 


184 


ELECTBICAL ENGINEERING TESTING 


used by varying the load on the synchronous motor and its 
excitation, and tabulate as in Test No. 70. 

(5) Plot the external characteristic curves having volts as 
ordinates and currents as abscissie for power factors differing by 
0-1 at a time between PO and the lowest obtained in the curves 
between voltage and power factor (3) to (4) above, each curve in 
(3) and (4) supplying one point only in each of the characteristic 
curves corresponding to the current and voltage, and the power 
factor for which the particular characteristic is plotted. 

Inferences. —State clearly all that you can deduce from the 
results of your tests. 


(72) Variation of Exciting Current with the 
Armature Current of an Alternator to 
maintain Constant Terminal Voltage on 
Inductive and non-inductive Loads. 

Introduction.— This test is a direct measurement of the range 
of variation in both the resistance and current of the field 
regulator required to maintain constant terminal voltage for 
any range of load, and which can otherwise be deduced from 
a reference to the external and magnetization characteristics of 
the alternator when available. 

Apparatus. —That detailed for Test No. 70, with the addition 
of an adjustable inductive resistance for combination with R. 

Observations. —(1) Connect up, as in Fig. 69, with R non- 
inductive only at first and the wattmeter inserted as a check 
on the product A X V. Level and adjust all instruments, which 
require it, to zero, and on starting up the motor alternator see 
that all lubricating arrangements are working properly. 

(2) With the speed and terminal voltage each adjusted to the 
normal value and kept constant—the former by regulation at 
the driving source, and the latter by field regulation of the 
alternator—first note the value of exciting current (a), on open 
main circuit, and then with S closed for each of a series of 
8 or 10 armature currents, A rising by about equal increments 
to full load by adjusting the non-inductive resistance R^ the 


ELECTRICAL ENGINEERING TESTING 


185 


field regulation of the alternator being adjusted so as to keep V 
constant with the speed constant. 

(3) Vrith R now inductive, and the same value of speed and 
voltage as in obs. 2 maintained constant, adjust the inductive 
part to its maximum value and vary the non-inductive part so 
as to maintain the main current A constant at about quarter 
full-load value, the field current being correspondingly varied to 
keep V constant. Note the readings oi A, V, W, and (a). 

(4) Repeat (3) for the same speed and voltage, but for 
constant values of A of about half, three-quarters, or full load, 
and tabulate as in Test No. 70. 

(5) Plot the four aaciliary curves (vide p. 182), having power 

/ ir\ 

factors, cos. ^ = jTpj’ oi’dinates, with exciting current (a) as 

abscissae, for each of the four constant values of A respectively. 

(6) By interpolation and transference, plot the desired relations 
between exciting currents (a) as ordinates, with main currents 
(d) as abscissce for non-inductive load (obs. 2), and for inductive 
load (obs. 3 and 4) at constant power factors of 0 9, 0‘8, etc. 

Inferences.—State clearly all that can be deduced from a 
study of the shape and relative dispositions of the curves in 
(6) above. 


(73) Determination of the Efficiency of an 
Alternator without running it on load. 

Introduction. —The preceding method, although a direct one 
for obtaining the efficiency of any generator by measuring the 
H.P. absorbed in driving it and the output in the usual way, 
requires some type of transmission dynamometer and has there¬ 
fore a limited application on account of the difficulty of measuring 
the large H.P.s in the case of large generators. While small 
alternators can be tested in this way, the method of driving the 
generator by an electromotor (preferably direct coupled) having 
a known efficiency-load curve, is more accurate, and also has 
a much more extended application in range of power. The 
power supplied to the motor x by its efficiency at that load 
= the power taken to drive the generator. 



186 


ELECTRICAL ENGINEERING TESTING 


Both methods are however costly of application in the case of 
the larger generators and an arrangement similar to Swinburne’s 
Test No. 82 would obviously be preferable in many ways. 

Further, if while absorbing (from an outside supply) only a 
small fraction of its rated full-load output, the armature of an 
alternator carries full load current at normal excitation, both 
the losses and temperature rise can be determined at an 
economical cost of energy consumed in the test. 

From the fundamental principle that, in any transforming 
device 

output = input — total internal loss 

we see that if the losses can be obtained in the various portions 
of the alternator, the input and efficiency are at once deducible 
and that the method is applicable to any size of machine, however 
large. 

The total internal loss in an alternator is made up of— 

(a) The total copper loss TF^ in the armature windings, 
thus— 

If Ca = the current per phase of armature winding 

and = the resistance per p)hase „ „ „ 

the total copper loss- Wq in the armature of a—single-phase 
alternator = C^r ^.; of a two-phase alternator = ^C^r^ and of 
a three-phase alternator = 

{h) The total power absorbed in excitation TF^. Thus if = 
the exciting current employed at a pressure of F^ volts, then 
F^. 

(c) The total power absorbed in mechanical friction Wmf and 
made up of—windage, bearing friction, brush friction. 

(cZ) The total power spent in magnetic friction made up 
of—hysteresis and eddy currents in field and armature core IF/^^. 

Then the total internal loss IF^ = IF^ + TF^ -}- TFj^^. + IFf^. 

The last two losses, which may be termed the stray power, may 
be determined experimentally at no load by running the alter¬ 
nator as a synchronous motor, light and unloaded, and 
measuring the power absorbed by wattmeter. 

Apparatus. —Alternator D to be tested, capable of being 
driven at no load and normal speed by some outside source of 
power. This might preferably be its direct-coupled exciter, if 


ELECTRICAL ENGINEERING TESTING 


187 


there is one, and provided that, when used as a motor, it is 
powerful enough, or, in the absence of this, a direct-coupled 
motor 31 of known efficiency. The necessary switches; a watt¬ 
meter having a range up to say 5% of the full load of the 
alternator; an ammeter A; and a voltmeter V in the case of 
single-phase alternators and of two- and three-phase alternators 
with equally-balanced phases. Two of each type of instrument 
will be needed for two- and three-phase machines out of balance. 

Observations.—(1) Connect up as shown in Fig. 71, adjusting 
such of the instruments as need it and assuming the alter¬ 
nator E to be a three-phase machine for instance. See that all 
lubricators feed properly. 



( 2 ) To run as a synchronous motor from some a.c. supply 
Q of the normal voltage Fand frequency developed by E. First 
start up E to its normal speed by means of 31 and adjust its 
exciting current ( 7 ^ so that its terminal voltage F that of the 
supply ^3 being open, but ^2 shorted by a wire and 
AS 3 by two synchronizing lamps (see p. 441) not shown, close this 
three-throw switch at the instant when the Ictmps ctve out and at 




























188 


ELECmiCAL ENGINEERING TESTING 


once open S. The alternator D will now continue to run as a 
synchronous motor at normal speed, frequency and voltage. 

(3) With D running as just mentioned, adjust its exciting 
current by the rheostat R so that a minimum intake current 
A is obtained and hence maximum power factor, and note the 
wattmeter readings Tl\ and and all the other instruments 
and speed. 

Then 

These losses are approximately constant for all loads, slightly 
increasing as the load increases due to change of induction 
througl/ increase of excitation for voltage regulation, and to 
armature reaction in the normal operation of the alternator. 

(4) With the same normal supply frequency as in obs. 2, and 

same excitation as found in obs. (3), increase the supply voltage V 
(by means of the field excitation of the supply Q) so as to 
obtain a series of supply currents A rising by about equal in¬ 
crements from the value found in obs. (3) to full load, and note 
the readings of ^ F and at each. 

Note. —The synchronous motor D will run throughout obs. (4) 
at constant normal speed but at a decreasing power factor, as 
was shown in Test No. 106, and since the excitation is constant 
at normal value, the losses {Wc-\- ^^if) measured by 

W^ and IFo will be the same as for the machine used as an 
alternator when giving the same current loads from its armature. 
The excitation loss is also known, and the total internal loss 
at all loads can therefore at once be found and the efficiency ^ 
obtained from the relation— 

^ output 

output + losses’ 

The accuracy of the values of efficiency (S) thus found will 
depend on the degree of accuracy with which the losses, in 
obs. 4, can be measured; tliis latter may not be high on account 
of the increasing inaccuracy of wattmeters on the lower power 
factors. 

(5) If necessary, find the rise of temperature of the macliine 
after a six hours’ run on full-load or other desired condition. 

Tabulate your results for the preceding test as follows— 




ELECTRICAL ENGINEERING TESTING 


189 


Alternator: No. . . . Typo . . . .Maker . ; . 

Full Load : Amps. = . . . Volts = . . . Speed = . . . Frequency = . . . 
Resistances: Armature per phase ?•„ == ... Field = . . . 

Wattmeter Constants A'j = . . . K 2 = . . . 


>> 

P< <u 
P. 3 
3 O' 

02 E 


Exciting 


£ 

o 


2 to 


CQ 

'o 

?> 

Apparent 
Watts V 3 A V. 

Watl meter 
Readings. 

«11^ 
fS tv M 

H ^ 

M ~l~ 

^ 1-4 

0 -te V 

^ 1 '”' 

S 1 

at 

l> 
g II 

P CO 

p-l 0 

0 

05 

J3 

0 

£ 

cs ^ 

S 

<1 

TFi. 










-2 II ^ 

p CO ~ 

43 

OQ 






Plot the efficiency-load curve of the machine considered as an 
alternator having values of efficiency ^ as ordinates, and values 

of VSAV (calculated for each value of A in the table but at 
normal voltage) as output at P.F. = 1. 

The separation of the losses can be obtained in much the same 
way as that indicated in Test ISTo. 77 for direct current machines 
by the use of the motor 31 in the following way— 

(4) With S^, /S'g, /Sg and both open, run the alternator at its 

normal speed and note the readings of a and v. If = 

the efficiency of 31 at this speed and load, then the 

power absorbed in mechanical frictions since with the fields of 
E unexcited there will be practically no iron losses. 

(5) Next close and adjust R so as to give normal full load 

exciting current. Vary r so as to obtain the same speed as 
before and note the readings of a and v. If 2/2 = the 
efficiency of 31 at this load, then 2/2^^-2T2 — + IP/r find hence 

the iron losses Wip = 2 / 2 ^ 2'^2 “ VNNv 

Having now obtained the iron and friction losses the 
efficiency can at once be deduced for all assumed loads. 

Tabulate your results for Tests 4 and 5 as follows— 


EfTiciencics of Motor M at Speed used : (at load aiVi)y\ = . . .; (at load . 


Alternator 

Speed. 

Alternator unexcited. 

Alternator Excited. 

II . 

to j::' 

rH 

V s 

1 — 1 
OJ ^ 

S ' 

J cs 

0 « 
p (N 

Reading of 

Friction 
Loss 
Wmf = 

1/lrtlVl. 

Reading of 

Friction + 
Iron Losses 
Wmf + 
WiF = 
y‘2a-2'C2- 

Ammeter (a) 

aj. 

Voltmeter (v) 

n- 

Ammeter (a) 
02- 

Voltmeter (v) 
V2. 







i 



















































190 


ELECTRICAL ENGINEERING TESTING 


The determination of the iron losses at any speed by the 
retardation method cau be undertaken in alternators having 
sufficient inertia or momentum in their moving portion to 
prevent them slowing down to rest too quickly for readings of 
speed to be taken at intervals. 

(6) With aS'p and Eje both open, run D at normal speed by 
means of M. Then at a noted interval of time, the speed being 
at normal value, open S and note the speed by tachometer at, 
say, \ minute intervals as the alternator D slows down to rest, 
the motor M being of course unexcited. The retardation in this 
test is due to Wmf> 

(7) Repeat (6) with \ \ and full load exciting currents 

Ce by varying R. The more rapid slowing down in this test 
is due to WjiiF + Tabulate your results as follows— 


Value of Wi + JF 2 - 3 A^ra (from Exp. 3). 


Alternator Unexcited.- 

Alternator Excited 

Values of 

Actual 
mean 
Power in 
Watts 
absorbed. 

Times 

^2 • • 

Speeds 

Til Tljj 712 • • 

Exciting 

Current 

. . 

Times 

t, ti, <2 . . 

Speeds 
n, ni, «2 . • 

Wk 

I 

Average 
Speed N= 
n -p wj . . 

2 










Plot curves for 6 and 7 between speeds as ordinates and times 

as abscissae, and between as ordinates and average speeds N 
as abscissae. 

The rationale of the retardation method is as follows—• 

Let I = the moment of inertia of the rotating system in 
C.G.S. units, or, gramme—cm.^ about the axis of rotation, and 
let IP be its angular velocity in radius per sec. Then* the 
kinetic energy of the whole system or energy of rotation 
Ke = ergs = where (?^) = speed in revs, per second. 


/27m\2 


Hence the kinetic energy = j / x 10 


= 548 10 72“/ Joules or Watt seconds, 

where (?i) is now in revs, per min. 

If now (?i) = the normal speed of the alternator at the instant 
t of opening s and . the successive speeds noted 

at times t-^, , from the instant (t) when slowing down 

commences. 























ELECTRICAL ENGINEERING TESTING 


101 


Then the energy expended or work done in the first interval 
of time in overcoming resistance is proportional to 

548 X 10-12/(,j,2_,,^2) 

n ^ 1. 1. 1 nr 548 X 10-i2/(«2_„ 2x 

and the mean power absorbed IKx=-watts 


t-L 


, ,7- n + n, 
at an average speed N = —-^ 


revs, per mm. 


TTv 


By plotting a curve between values of -j- for successive 


speeds and intervals as ordinates and the corresponding average 
speeds N for successive pairs of speeds as abscissa3, we can get 
the loss at any speed, and that at normal speed by the point of 
intersection of the curve (produced backwards) with the full 
speed ordinate. 

Note.—Since / is a constant but unknown quantity, the 


ordinates of the curve are the values of 


I 


and the ordinates 


do not therefore represent actual watts, but = watts a con¬ 
stant (7). 

From Observation 3, however, we know that this normal 
speed ordinate = + TTg — 3 ^ 2 ^^^ the total friction and iron 

losses. Hence the value in actual watts of any other ordinate 
corresponding to any other speed is at once found by simple 
proportion. 

From the above data compile the following general table—■ 


Alternator: No. . . . Tj'pe . . . Maker . . . 

Full Load : Amps. = . . . Volts V = . . . Speed = . . . rrequency= . . . 
Resistances: Armature per phase Va = . . . Field rs = . . . 


Main Output 
Current 
Assumed C. 

Watts Output 
Wo = VO. 

Losses. 

s; 

Efficiency 

Excitation 
CeVb= Ws. 

Armature 

3 (7«2ra= Wc. 

Iron and 
Friction 
Waif+ Wif 
= w. 

T9tal Wl^ 
Wc -k Wb + w. 










Plot the load-efficiency curve having efficiency as ordinate and 
Wq as abscisste. 
























192 


ELECTRICAL ENGINEERING TESTING 


(74) Efficiency and Internal Loss Test of a 
Pair of Alternators (by the Hopkinson 
Principle). 

Introduction, —The following method ia available when two 
similar alternators, as nearly alike in output as possible, are 
obtainable. It is analogous to the Hopkinson test of a pair 
of D.C. dynamos, and has the double advantage that all the 
measurements are electrical ones; and also that while both 
alternators run under load conditions, so far as field and arm¬ 
ature current is concerned, each is running at only a fraction 
of its full K.W. capacit}^ and consequently the power taken 
from the necessary outside supply is small, even in the case 
of the testing of large alternators. The method further lends 
itself most conveniently to the determination of the tempera¬ 
ture rise of each machine under the same heating conditions as 
would obtain if each was run for, say, six hours at full-load 



output, but with far greater economy in the cost of energy 
consumed from the outside supply. 

The test can be carried out with the two alternators under one 
or other of two conditions, viz. (1) when they are not mechanically 
coupled together, or (2) when their shafts are in accurate ali^-n- 
ment and rigidly coupled. In each case (a) the alternator to be 
used as a generator (say must be either belted or (prefer¬ 
ably) direct coupled to a small direct-current motor J/, bavin" 
a known “efficiency-load” curve at the speed to be employed; 
(6) the second alternator must be electrically connected to 
B^ and run as a synchronous motor from its supply; (c) an 























ELECTRICAL ENGINEERING TESTING 


193 


additional driving source of power (which might be another 
D.C. motor J/g) will be needed to run into synchronism, and 
afterwards to be disconnected if possible. Now, although under 
condition (1) above, the power factor of the circuit between 
and Z^g will decrease as the circulating current increases, 
any error that might be introduced from this cause, and referred 
to in test No. 73, p. 188, is eliminated in the present test, as the 
losses are now measured in the D.C. circuit of J/j instead of 
being obtained from the readings of and TFg. 

Under condition (2) above, however, in which the shafts of 
I)I and Z>g are rigidly coupled, the P.F. of the circulating 
circuit remains constant for all currents with any particular 
bolting of the half couplings. If this bolting can be varied, 
then each constant value of the P.F. can be varied fi'om unity^ 
when the half couplings are bolted so as to make the E.M.F.s 
of and 2) differ by 180° in phase {i.e. when in direct op¬ 
position of phase), to zero^ when one half coupling is bolted with 
an angular difference relatively to the other half coupling equal to 
the angular pitch of the alternator field. Throughout this range 
the alternator with the greater excitation will be acting as 
generator. 

Obviously with the rigid coupling of condition (2), the two 
alternators must not only be similar in output and voltage, but 
must also give the same frequency at the same speedy whereas 
in condition (1) their frequencies can be different if necessity 
arises. Further, the general applicability of the present method 
is questionable, e.g. in condition (2) the alternator must be 
coupled either (on the left) up to or (on the right) up to the 
other end of so that either or must have a shaft 
extension each end. On the other hand, the test under con¬ 
dition (1) will need a second driving-motor which for the 
highest accuracy should be capable of disconnection from 
after this is synchronized. These facilities may be obtainable 
in certain works, but seldom exist in a college laboratory. 

Apparatus. That depicted in Fig. 72, where RiR^ field 
regulators for adjusting the exciting currents in the fields 
of 

are starters or main circuit adjustable rheostats for the 

motors 


o 


194 


ELECTRICAL ENGINEERING TESTING 


XjLg synchronizing lamps made for voltages equal that 
per phase of the supply. 

jE* is a D.C. supply, and three-phase alternators are assumed 
for test as presenting slightly greater complication in connection 
and test than single or two-phase machines. 

Observations. —(1) Connect up similarly to Fig. 72, but with 
any modifications which the facilities available in machines 
necessitate. Level and adjust to zero all instruments needing 
it, and on starting up see that all lubricating arrangements feed 
properly. 

(2) With running at normal voltage V and frequency, 
synchronize by obtaining equal voltages as V.V., and closing 
SSS at the moment when L^L^ are definitely out. d/g (if used) 
being disconnected electrically and mechanically, if the latter is 
possible. 

(3) Next adjust so as to make A a minimum for constant 
nomnal values of both V and the frequency. 

Note the readings of all the instruments and the speeds of 
and D^. 

Then the output of il/j, or power required to drive 

a ^ 

= = the total internal running losses ( Wc + Wmf + ^^if) 

(see p. 186) in and Z >2 together (excluding excitation losses 
in 

Where e = the efficiency of at this load and speed (from 
curve), 

O V 

= the losses in either alternator. 

(4) Now reduce the excitation of by the same amount as 
that of is increased, so as to obtain a series of main 
circulating currents A, rising by about equal increments up to 
the full load current of the machines, and note the readings of 
all instruments at each value of current A. Then the increased 

O/ ^ 

value of = the losses in either alternator at the respective 

current values A, where (e) has an increasing value at each 
load as taken from the efficiency-load curve of d/j. Adding half 
the total excitation loss, viz. + ^2 ^ 2)5 the above loss, 

we get the total internal loss Wl (p- 186) corresponding to each 





ELECTRICAL ENGINEERING TESTING 


195 


of the current and which will he practically those which 
would exist if either machine was supplying those currents as 
an alternator at the same speed and voltage. Tabulate all your 
results as follows :—• 


Alternator Di: No. = . . , Type . . . Maker . . . Armat. Ees. per phase ra = . . . 

Full Load: Amps. = . . . Volts Fs = . . . Speed = . . . Frequency = . . , 

Field Res. 

Alternator D 2 : No. = . . . Type . . . Maker . . . Armat. Res. per phase ra = . . . 

Full Load: Amps. = . . . Volts F, = . . . Speed = . . . Field Res. 

Wattmeter Constants A'j = . . . A '2 = . . . 

Driving Motor Mi: No. = . . . Full Load : Amps. = . . . Volt.s. = . . . Speed = . . . 



(5) Plot the efficiency-load curve of either machine considered 
as an alternator having values of 5 as ordinates and values of 

load V3A Fs as abscisste. 

Inferences.—Clearly state all that can be deduced from the 
results of the test. 

Note. —If the wattmeters and ITg (which are not really 
essential to this test, and only useful, if available, for observing 
and comparing certain quantities) are omitted, the eight columns 
in the table, necessitated by their use, can also be omitted. 


Determination of the Distribution of 
Potential round the Commutator of a 
Dynamo. 

General Remarks.—On considering the action which occurs 
with a single turn of wire on a coreless armature as it rotates at 















































19G 


ELECTRICAL ENGINEERING TESTING 


a uniform rate through one revolution we find that, starting from 
a position 0", which may be termed the zero position, when its 
plane is perpendicular to the direction of the lines of force due to 
the fixed field NS (Fig. 73), its E.M.F. is 0, because it is slipping 
through and not cutting these lines. When it gets to 90°, the rate 
at which it cuts the lines is a maximum, and this decreases round 
to 180° again, when the E.M.F. is 0, and after then the effect is 
simply repeated. The zero position nn is the neutral axis or 
diameter of commutation for no current in the coil or armature, 
while RR is the line of resultant magnetization at right angles to 


\ I 



Fig. 73. 


nn. In fact, the E.M.F. generated in the coil at any position is 
approximately cc sine of the angle of rotation from nn, and, as 
■we have seen, is zero at 0° and 180°, and a maximum at 90° and 
270°. If the coil be wound on an iron core, carries a current, and 
is made to rotate, it will react on the fixed field NS, causing a 
distortion of this latter, so that will now be the neutral 
axis or diameter of commutation and R^R^ the line of resultant 
magnetization. In other words, the resultant field produced by 
that due to the armature and field NS will be forced round 
through an angle ROR^ in the direction of motion, and will cause 
the brushes to advance through an equal angle non^ to the 








ELEGTRIGAL ENGINEERING TESTING 


]97 


position which angle is called the angle of Lead'^ of the 
brushes. 

If now, the circular path of the coil, which we will assume for 
the moment not wound on an iron core, is developed out into 
a straight line AG, and the sine of the angular position from 
0 {i.e. A), Fig. 74 I., plotted on the ordinates at each such 
position, the curve APBQG will be obtained, showing the 
variation of E.M.F. with angular position in one revolution. 
Thus .4 and C correspond to 0° or position nn (Fig. 73) when the 


P 



Q 


P 



E.M.F. is nought, while PD and QE correspond to 90° and 270°, 
or position RR when E.M.F. = maximum. This curve is called a 
sine curve, and it possesses the uniform shape shown in Fig. 74 I. 

Now in an ordinary iron core armature the E.M.F. of each 
coil fluctuates in a manner similar to that shown in Fig. 74 I. in a 
bipolar machine, and to that shown in Fig. 74 II. in a multipolar 
machine, but the commutator commutes such E.M.F.s so as to 
develop an E.M.F. at the brushes perfectly continuous in direction. 

Considering the armature as a whole, the line nn or i. e. 
the brushes of the mo,chine divide the armature coils into two 









198 


ELECTRICAL ENGINEERING TESTING 


halves, which are in parallel with one another, now each half 
consists of separate coils in series with one another, each giving a 
certain but different E.M.F. depending on their position relatively 
to 0“ or nn. These E.M.F.s being in series are added together in 
each half and the two summational E.M.F.s put in parallel. 
Thus the E.M.F. between the brushes = sum of E.M.F.s round 
one half of armature between those brushes. Consequently, as 
we proceed from, say, the negative main brush, the E.M.F. (if we 
could sample it) round either half increases up to the other main 
brush, first slowly, then rapidly, and finally slowly again when 
nearing the maximum point. 

From the preceding remarks it will be evident that two 
investigations can be made on the E.M.F. of arrnature coils— (a) 
that of any one coil in different positions of a revolution; (b) the 
way in which the E.M.F. varies as we proceed from one brush 
right round the armature. 

There are many methods of performing these investigations, 
and amongst those most easy of application in practice may bo 
mentioned Prof. S. P. Thompson’s, Mr. Mordey’s, and Mr. Swin¬ 
burne’s, and these we will now consider in detail. 

In Thompson’s method of operating investigation [a) above, the 

E.M.F. of a single section on the 
armature can be sampled at different 
points in the revolution. The ar¬ 
rangement consists of two flat metal 
strips or brushes b^b^ (Fig. 75) fixed 
to a piece of wood at a distance apart 
equal to the width between two con¬ 
secutive commutator bars. A volt¬ 
meter is connected across b^b^, which 
therefore measures the E.M.F. of a 
single section of the armature wind¬ 
ing which is passing through the 
particular position of the field, cor¬ 
responding to the position of the contacts. It is preferable 
that the compound brush bj)^ should be mounted on a brush 
rocker capable of swivelling round the shaft over a degree divided 
scale, so that angular distances from some starting-point may be 
accurately obtained. 






ELEGTRIGAL ENGINEERING TESTING 


199 


The method has the adv^antage that only a comparatively short 
range accurate reading voltmeter is needed, say, to about ten 
volts or so in the case of a 100-volt machine. The main brushes 
E^R 2 must be arranged to allow to pass them on the 
commutator. 

The readings of the voltmeter will be different according to 
whether the machine is giving no current at all or its full-load 
current. If the machine is shunt wound it may in the former 
case be self-exciting, as the shunt current will be so small 
compared with the load current as to not affect the distribution 
round the commutator. 

If now the readings on V are plotted on the ordinates of a 
curve with the corresponding angular positions right round the 
commutator on the abscisste, the curve will not only show the 
variation of E.M.F. of the coil, but will show also the distribution 
of the magnetic field in the air gaps, the best position for the 
brushes and the “ a7igle of Lead ” which must be given to these 
when running on full load due to the shifting round of the 
resultant magnetic field RR (Fig. 73) to Rfi^ 


(75) Determination of the Distribution of 
Potential round the Commutator of a 
Dynamo. (Mordey’s Method.) 

Introduction.—The following is a convenient and simple 
method of finding the above-named distribution, and consists in 
measuring the potential between one of the main brushes and a 
single movable or Pilot brush capable of swivelling right round 
the commutator. It is then found that the potential increases 
or decreases from that main brush (according to whether it is the 
negative or positive one) round each half of the armature to the 
other brush, and that the variation is regidar in a well-designed, 
but irregulai' in a badly-designed machine. 

To represent the resulting variation or distribution graphically 
Prof. S. P. Thompson proposes drawing a circle OBC (Figs. 76 
and 77) to represent the commutator and divide it into, say, 36 
equal parts of 10° each, set off radially outwards from the circle, 


2C0 ELECTRICAL ENGINEERING TESTING 


lines oc potentials at the various angular positions of the pilot 
brush, thus getting the outer or potential curve OAI), 


P 



0 


Fig. 76. 


Next obtain the developed diagrams to the right of Figs, 76 and 
77 by laying off a horizontal base to represent the length of 
the circumference of the circle OBG, then at the proper points 
along this angular line set up the radial lines from the left-hand 
Figure, due regard being paid to sign. 

Fig. 76 is the result obtained with a well-arranged dynamo, 
and Fig. 77 with a badly-arranged one. 



0 


Fig. 77. 


These curves show us several points, as, for example, the steep¬ 
ness of the curves in the right-hand diagrams enable an idea of 
the relative activity or idleness of the coils in these positions to 
be obtained, also the position of the brushes to give the best 
result and the distribution of field in the air gaps. 













ELECTRICAL ENGINEERING TESTING 


201 


Fig. 77 may result from a machine in which the pole pieces are 
badly shaped, or the brushes badly placed. 

Apparatus, —Dynamo to be tested, fitted with a third brush or 
pilot brush F capable of swivelling round the whole circle 
divided into degrees, and of making contact on the commutator 
at any position ; a rather long range accurate reading voltmeter F, 
and arrangements for taking a load from the machine when 
required. 

Observations.—(1) Calling the two main brushes and B^ 
and the pilot brush F, connect F between the negative main 
brush and F, the dynamo being shunt wound and excited off its 
main brushes B^B^. 

(2) Run the machine on open external circuit at normal speed, 
and adjust F in line with the negative brush, which latter has 
been previously adjusted to give no sparking. 

(3) Note the reading on F and the degree scale of P, and 
repeat every 10° right round the commutator at constant speed. 

(4) Repeat 2 and 3 for a full-load current taken from B^B^^ 
adjusting the speed (constant) to give the same voltage as 
before, 

(5) Repeat obs. 1-4 with the same machine run as a motor. 

(6) Tabulate your results in a convenient form, and plot a pair 
of curves for each test in the way indicated above. 

Inferences. —State very clearly what you can deduce from 
your curves of distribution,"and indicate in the developed diagram 
the positions of the poles, brushes, and resultant magnetic field of 
the machine, 

(76) Determination of the Distribution of 
Potential round the Commutator of a 
Dynamo. (Mordey-Swinburne’sMethod.) 

This is a neat modification of the preceding method, and con¬ 
sists in connecting a high resistance wire F^Fg across the main 
brushes B^B^, and finding by means of a sensitive detecting 
galvanometer G a position G along the resistance F^Fg such that 
G does not deflect. The point C is then at the same potential as 
P; hence since F^Fg is fixed, the distance ViG or V^G gives 
the relative potentials for various positions of P. The potentio- 


202 


ELECTRICAL ENGINEERING TESTING 


meter can be easily calibrated by taking one single 

reading of the volts (V) across the distances V^C, for 



VC 

instance, in volts = ^ 


of V 


whore 7^0 and F^Fg 

ohms, say, or some known 

units. 

The speed must be constant 
throughout the test, so as to 
maintain F constant. Being 
a zero method it is very ac¬ 
curate, and has the advantage 
of not requiring a voltmeter 
which has to be equally accur¬ 
ate over its whole range, but 
only at one point. 


(77) Analysis of the Total Internal Loss of 
Power in Direct Current Dynamos and 
Motors. 

1 

Introduction. —In test No. 82, p. 220, it is pointed out that the 
total internal loss of power in a direct current dynamo or motor 
is made up as follows— 

(i) Chopper Loss occurring in the armature and field coils, caused 
by heating due to the passage of the current. 

This is at once easily calculable from the relations there given 
for finding the copper loss in either series, shunt or compound 
machines, when the resistances of the several coils, and the 
respective currents which each carries, is known. The loss in 
each circuit varies as the square of the current. 

(ii) Mechanical Friction due to air churning or resistance, 

brush and bearing friction, each of which varies as the speed 
simply. 

(iii) Eddy Current or Foucault Current loss occurring in the 
armature core, and also in the armature conductors, and varying 
as the square of the speed for the same excitation, since the 
eddy currents will be directly cc speed at constant excitation 
while the watts used in producing them will vary as the square 
of these currents, or if IF^; = Watts 'svasted in eddy currents 












ELECT RIGA L ENGINEERING TESTING 


203 


and 71 = speed in revs, per min., then loss from this cause is 
IFjs; X where Ke is a coefficient depending on the eddy 

loss. 

(iv) Magnetic Hysteresis in the core due to reversals of mag¬ 
netization in it as it rotates and x to its speed. Jf W^L = the 
loss from this cause and its co efficient, then Wji x nK^ ] 
hsnce the total iron loss IF/ = We -f We ~ nK^ + u^Ke- 

This equation has been made use of in several methods for 
separating these losses. Thus in Mr. Mordey’s method, which is 
applicable to determining the losses in an unwound armature 
core as well as a wound one, the armature to be tested is driven, 
when in position between its own field poles, at different speeds 
(n), with its field (a) unexcited, (d) excited to a constant degree, 
(c) excited to various degrees, by an electromotor, and the power 
so required measured by a dynamometer or by knowing the 
efficiency of the motor accurately. 

On plotting a curve between the speed (n) and the powers W 
required to drive at different speeds in a constant field, the 
constants Kjj- and can be found from it. 

Mr. Kapp’s method is a slight modification of the preceding, and 
is only applicable to a ready-wound armature core. It consists 
in measuring the power IF required to run the armature to be 
tested at different speeds in a constant field N, by running the 
armature itself as a motor “ light,” and noting the corresponding 
voltage V and current A taken at each speed { 71 ). 

If then Ta = total number of armature turns all round we 
have the fundamental relation V = T^Nn 10"^. 

But W=AV = AT^ Nn lO’S = nKj^ + n 

jr 

A = -— 4- n - - —-o = (a constant -f w x a factor). 

T^NIO-^ 2\N10-^ ^ ' 

On plotting therefore the curve between A and n to the axes 
OY and OS with (?^) along we shall obtain the straight line 
PQ. The ordinate OP is x current required to overcome fric¬ 
tion and hysteresis, while tan. 0 x the eddy current effect. If 
OP is plotted to a scale of current, then Kjj = 0 P,NTa\W^i 

when F — P is also known. 


We also have 



Thus the three separate 


Hysteresis -h Friction 
(Hysteresis + Friction) + Eddies, 
factors or losses are each determined. 









204 


ELBGTRIGAL ENGINEERING TESTING 


The following graphical method of separating the various 
losses is a simple and convenient one, and independent of any 
mathematical treatment. It is due to Mr. R. H. Housman, and 
is as follows— 

Separately excite the field magnets to the normal amount and 
keep this constant. Note the current and speed of the armature 

when running light as a motor 
for different noted voltages 
applied to it. Plotting current, 
which a: torque with given 
field on the ordinates, and volt¬ 
ages which cc speed with 
given field on the abscissae, or 
Joules per revolution on the 
ordinates and revs, per second 
on abscissae, the straight line 
PQ (Fig. 19) is obtained cut¬ 
ting the current axis in P. 

If Q is any point on PQ and 
QS is parallel to OP, then the tokvl loss for that speed OS is 
given by QS x SO. If PR is parallel to OS, then the area 
PS cc OS cc power lost in hysteresis and friction together, and 
area QE x RP cc OS^ cc power lost in eddy currents where QP 
cc RP cc OS. Repeating the above with a different excitation 
will give a second line P'Q\ usually parallel to PQ, showing 
that the eddy currents are constant for a given voltage. 

It may be noticed that the total loss corresponding to any 
point such as Q on PQ = product of co-ordinates = OS x QS, 
and not the area of the Fig. POSQ. In other words, the Fig. 
represents the nature of a dynamo Characteristic rather than the 
indicator diagram of a steam-engine. 


B 

D 



1 

1 

-^1 

Currents 

1 

1 

1 

1 

iP 

Hysteresis 

\n 

1 

1 

1 

Brush 1';^ 

Bearing & Wind 

1 


Fig. 79. 


To obtain the total mechanical friction losses, run th-e arma¬ 
ture with brushes down, field disconnected and unexcited by a 
direct coupled motor, and note the increase of current required 
to drive over that needed for the motor alone. Plotting this 
current OR on the ordinates and drawing BG parallel to OS, the 
area OC cc total mechanical frictions, and .*. BR must be cc 
to the hysteresis loss alone. On noting this excess driving 
current with the brushes up, we get OB, and finally the area 
OE cc bearing and wind friction only. BG being oc the brush 
friction alone. 








ELECTRICAL ENGINEERING TESTING 


205 


The total losses for a given voltage will be a minimum for a 
certain induction in the armature core, usually between 15,000 
and 16,000 lines per square c.m. Since the hysteresis losses 
increase rapidly with increase of field, while the frictional losses 
increase with decrease of field due to the higher speed needed to 
obtain the same voltage. 

For high inductions up to 18,000 or 20,000 the eddy currents 
cause the curve to bend upwards, and also the angle 6 to be 
greater. Thii is probably due to the eddies generated by the 
stray leakage field through the shaft, etc. If the line PQ 
bends, it shows that the eddy-current losses are producing per¬ 
ceptible demagnetization on the field. Since both the eddy and 
hysteresis losses increase with armature current, these losses 
should really be measured with full-load armature current 
flowing by using the method of Fig. 86, which with careful 
adjustment of excitation will give considerable range of speed 
for constant armature current. 

This question of the separation of the various losses is of 
great importance to the dynamo maker, enabling him to see 
in what way a machine is faulty, i. e. whether the eddy-cur¬ 
rent loss is excessive due to insufficient lamination, or the 
hysteresis too great due to too hard or inferior quality of iron. 
We will now consider a complete experimental analysis in 
detail. 

Apparatus. —Exactly the same as that prescribed for test 95, 
and in addition an auxiliary motor should be available for coup¬ 
ling direct to the machine to be tested. 

Observations.—(1) Carry out observations 1-3, test 95. 

(2) Kepeat 1 for an excitation 25% above and 50% below the 
normal. 

(3) Disconnect all apparatus from the machine tested, and 
also the field from the armature. Connect the instruments up 
with the auxiliary motor, so as to measure the power taken to 
drive it. Demagnetize the field magnets of the motor to be 
tested by sending round the field coils a gradually diminishing 
(to 0) alternating current. 

(4) Measure the voltage and current needed to run the aux¬ 
iliary motor at some ten different recorded speeds between 0 and 
the maximum allowable. 


206 


ELECTRICAL ENGINEERING TESTING 


(5) Direct couple the auxiliary motor to the armature tested, 
and with the brushes down, note the new power given to the 
auxiliary to drive the two machines at some ten different speeds, 
the field of the machine under test being entirely disconnected 
and unexcited. 

(6) Raise the brushes and repeat 5, tabulating all your 

results as follows— 

Namk . . . Date . . . 

Motor tested : No. . . . Resistances: Armature = ,.. Ohms @ °C. Normal Voltage = ... 

Maker ... „ Shunt = ... „ „ „ ,> Amps. =... 

Type ... ,, Series = ... „ „ „ „ Speed =... 

Total copper losses = ... 


Speed in 
Revs, per 

Tested Motor Self- 
driven (lig^t). 

Auxiliary 
Motor alone. 

Motors 

collided 

Brushes 

down. 

Motors 

coupled 

Brushes 

up. 

Friction losses. 

Min. 

Sec. 

Excitation. 

w 

o 

> 

Amps. A. 

Watts 

(Total) 

CO 

'o 

> 

Amps. C\. 

11 



II 



11 

o3 ^ 

4^ 1 

o ' 

Bearing 

and wind. 

Wz-Wi. 

o Si 
-Cl 1 

g.C-J 

f ■ >4^ 




















(7) Plot all your results to the same pair of axes, having in 
each case the speed in revolutions per second on the abscissae and 
the power in Watts required to be given to the shaft of the 
dynamo under test to produce those speeds under the various 
conditions mentioned in observations 1-6 on the ordinates. 

(8) Calculate the various losses at normal speed as a per¬ 
centage of the total loss in the whole machine at full load. 

Inferences. —State very clearly all that can be inferred from 
your experimental results. 

Note. —A variation of the preceding method for measuring 
the hysteresis and eddy current losses consists in measuring 
the watts absorbed by the armature in running the machine as 
a motor light at a series of excitations between 0 and the 
normal, the speed being kept constant at normal value by adjusting 
the volts on the armature by means of a main circuit rheostat 
in series with it. 

Plotting a curve with armature watts as ordinates, and excita¬ 
tion as abscissae, we find its lower portion to be nearly straight, 
and this part produced to cut the ordinates will give the watts 
which would be absorbed at zero excitation, Thus the differ- 
















































ELEGTFJCAL ENGINEERING TESTING 


207 


ence between the watts at any given excitation, and at this 
zero value, will ba the power lost in hysteresis, eddies, and 
mechanical frictions. 

Again, if the curves are plotted between Joules per revolution 
as ordinates, and revs, per sec. as abscissae, the friction line BG 
separating frictional and electro-magnetic losses has a fixed 
position in the diagram ; whereas, with the axes denoting current 
and volts, a different friction line has to be drawn for each 
excitation, thus making it difficult to see what proportion of 
the whole loss is electrical and what frictional, when more than 
one set of curves corresponding with dififerent excitation is 
drawn on the same curve-sheet. 


(78) Measurement of the Coefficient of 
Magnetic Leakage v,” and of the 

Relative Distribution of the Waste 

Field of Dynamos and Motors. (Ballistic 
Method.) 

Introduction. —The present test has a most important bearing 
on the design of the magnetic circuit of a dynamo or motor, for 
since only a fraction of the total number of lines of magnetic 
force, generated by the field magnets, are usefully employed 
in cutting the armature conductors and so generating the 
requisite E.M.F., the results of the test enable the designer to 
allow for this discrepancy, providing he knows the coefficient 
of magnetic leakage “ v ” for the particular form and type of 
machine in question. 

In addition to this, the relative distribution of the waste field 
around the machine enables defects in the design of the magnetic 
circuit to be seen and corrected, for at the best the magnetic 
circuit of a dynamo or motor is very imperfect. 

It should be remembered that leakage of magnetic lines of 
force will take place across any two points between which there 
is a difference of magnetic potential, the magnitude of which 
leakage will depend directly on this potential difference, and 
inversely on the magnetic resistance of the path. 

The following is a convenient method of measuring or comparing 
the relative amounts of leakage in different parts of a dynamo. 


208 


ELECTRICAL ENGINEERING TESTING 


and therefore the static leakage coefficient v for the machine; 
the term static being here used to denote the value of v obtained 
when the armature is at rest, for it is well known that an 
armature delivering current exerts a demagnetizing action on the 
field which directly promotes leakage. Assuming the normal exci¬ 
tation constant, the leakage will increase with the output, and it 
will largely depend on the degree of saturation of the iron and on 
the relative magnetic reluctances of the various parts. The method 
depends on the measurement of induced currents produced by 
moving either (1) an exploring coil so as to cut the field to be 
tested, or (2) the field in such a way as to cut the coil, the latter 
method being here adopted. Either the relative or absolute 
numerical values of the stray and useful flux in the various parts 
can be found, the relative values being obtained with reference 
to that part in which the flux is a maximum which can be taken 
as unity. Knowing these, the absolute values can be obtained 
by running the armature at a known speed and measuring the 
E.M.F. without allowing it to develop current and thereby 
distort the field. The useful armature flux can now be at once 
calculated, and from it, that in each of the various parts, or 
thus:—suppose we have a circuit consisting of a ballistic gal¬ 
vanometer, resistance box, earth inductor of turns, mean 
area A^ square c.ms. in series with an exploring coil of Ag turns, 
mean area A^ square c.ms. w^ound round the magnetic field to be 
tested. If now the inductor, with its plane vertical or horizontal, 
is rotated rapidly through 180°, cutting the earth’s field of 
strength then the total quantity of electricity set up in the 

. ^ 2N.A.F, 

transient current is — = K sin. ^ where = ballistic 

constant^ = total circuit resistance, 0 .^ = angular throw in 
degrees. If the exploring coil is now made to cut the field to be 
tested of strength by suddenly making, breaking, or reversing 


the exciting current, we get = 


2 


IL 


= K sin. O 2 where 
•. Dividing we get 


and A 2 have the same meaning as before. 

r, 77 'iN.A.R^ c?9 .,11 

T 2 — ^1 j X ~ lines per square c.m. in the loop or search 

coil (in absolute measure) where d-^ and 6^^ = scale deflections 
corresponding to 6 ^ and 





ELECTRICAL ENGINEERING TESTING 


203 


As, however, it is the total field [A^F^ which we really desire 
to obtain, and denoting this by 

we have Fj, = ^ lines. 

Apparatus. —Earth inductor E ; resistance box R ; charge and 
short circuit hey A"; ballistic mirror galvanometer G (p. 569), having 
a small log decrement and periodic time about 8 or 10 seconds, so 
that this may be large compared with the time of flow of and 
(?2 which can therefore pass through the coil before it begins 
to move. A shunt wound dynamo to be tested; ammeter A ; 
rheostat(r) (p. 599); quick break switch E; and sourceof currents. 



Observations. — (1) Adjust the needle of G to zero, and wind a 
single complete turn of wire on the dynamo at position A, 
connecting it up with the other apparatus as indicated in Fig. 
80. The F.M. coils must be disconnected and excited separately 
from E. 

(2) Close E and adjust (?•), so as to get normal excitation 
through the F.M. coils. 

(3) Close Li, open E, and adjust R by trial so as to get a 
convenient throw on G, then note its value (E^) on breaking, 
and ( 7 ) 2 ) on making circuit at E, the excitation being that in 2. 
Repeat this twice and take the mean of each, calling it (d^}^ 

P 





































210 


ELECTRICAL ENGINEERING TESTING 


(4) Repeat 1-3 for each of the positions of the exploring 
loop indicated by the letters B, C, D, E, E, II, J, K, and L, 
respectively. 

(5) Repeat 4 for excitations 50% higher and 50% lower than 
the normal, and in each case calculate v from the formula, 

_ Total Field 
’'“Useful Field 

(6) Let down the brushes and run the machine at a known 
speed, measuring the E.M.F. E at each of the three excitations 
used, and tabulate as follow^s—• 


Name . . 

iV] = . . . turns 
= . . . sq. c.ms, 

Galv. resistance g = . . . ohms. 

Inductor resistance rg = . . . olims. 

Total resistance Ri = . . . oliins. 


Date . . . 

= . . . C.G.S. units. 
cZj = . . . Scale divs. 

Total No. armature conductors C = 
speed = . . . revs, per min. 

= . . . revs, per sec. (n). 


p, 

o 

o 


a 

o 


in 

O 


o 

o 


in ^ 


o 

c 

cS 

u 

X 

o 

CQ 


o 

P 

o3 

4^ 

^cn 

*w 

u 




o 


“ ” 
tx- 

•S s 

-t-3 ci 

X ? 


Throw on. 


to 

a 


CD 

f-i 


60 

R . 

'n 


4^ 

t. 

o.-t2 + 
in * 

pR o rH 

" ^ II 


a 


<3 




ii 

te; 


« o 
p p< 

S 






y. 

p 

& 

s 



Inferences. —State clearly all the inferences which can be 
drawn from the results of the above experiments, and point out 
their bearing on the design of field magnets for dynamos and 
motors. 



































ELEGTRIGAL ENGINEERING TESTING 


211 


(79) Magnetic Characteristic of a Dynamo 

with varying Air Gaps. 

Introduction. —It is of considerable importance, especially in the 
design of (fynamos, to know the effect which the length of air 
gap, between the field magnet (F.M.)pole faces and armature core 
has on the excitation required to force a given number of magnetic 
lines of force through the core. For convenience the curve show¬ 
ing the relation between the amp.-turns (A.T.) or magneto-motive 

force (M.M.F.) which = ^xA.T. in the F.M.s and the total 

useful flux of lines (A) through the armature will be called the 
Magnetic Characteristic for the air gap used. The flux (A) can 
be found in two ways : (1) by using a ballistic galvanometer in 
series with a “ search coil ” temporarily wound on the armature 
and noting the throws produced on the galvanometer by making, 
breaking, or reversing known currents in the F.M. coils; (2) by 
running the armature mechanically and noting its E.M.F. speed, 
and number of conductors round periphery, A being then calcu¬ 
lated from the fundamental formula A = A?^6'-^10^. This is the 
best and more practical method to employ, because the armature 
will now exert a slight demagnetizing action on the F.M.s tending 
to increase leakage and approximate more nearly to actual working 
conditions. The Exp. is divided into three distinct parts, viz. the 
determination of the relation between—■ 

(a) The M.M.F. and flux (A) through armature with constant 
air gap. 

[P) The air gap and flux (A) through armature with constant 

M.M.F. 

(y) The air gap and M.M.F. through armature with constant 
flux (A) in armature. 

Apparatus. —The dynamo D capable of being driven mechanic¬ 
ally ; tachometer ; voltmeter V ; ammeter a ; switch S ; rheostat 
r (p. 599); supply of electricity. 

The machine D to be tested must be specially constructed in 
order to be able to operate this test. As shown in Fig. 81, the 
pole pieces are each capable of being made to approach or recede 
from the armature by turning a massive screw bolt b fitted to 


212 


ELECTRICAL ENGINEERING TESTING 


each, by means of a suitable key. The distance apart of the pole 
tijxs can be read off on a scale C fixed to the body of the machine. 

Note.—The pole tij'is must never be closer together than the 
tioo zero scale divisions which will be termed their normal position 

in what follows, and must 
always be left <it this dis¬ 
tance after the test is over. 
To increase this distance 
turn the screw clock-wise. 
It will be noticed that the 
initial slopes of the curves 
in (a) are determined by 
the air gap, also that the 
air gap causes the curve to 
bend over. 

All lubricators must feed 
properly before the ma¬ 
chinery is started. 

Observations. — a — (1) 
Connect up as shown in 
Fig. 81, and adjust the pointers of all the instruments to zero. 

(2) Set the pole tips at exactly the noi-mal distance apart and 
adjust the speed so that with the maximum excitation allowable 
in the F.M. coils 25% above normal, the E.M.F. can be read off 
on V. 

(3) With air gap and speed constant, adjust the excitation to 
about ^ of the maximum allowable. Note this reading A and 
that on (v) viz. E. 

(4) Repeat 3 for about eight ascending equal increments of 
current to about 25% above the normal excitation. 

(5) Repeat 3 and 4 for the pole tips half-way and the farthest 
apart. 

(6) Repeat 3-5 for the same current values descending. 

(7) Plot curves in each case with M.M.F. as abscissae and N as 
ordinates. 

/?—(1) Adjust the exciting current to the normal value and 
the speed so that the E.M.F. can be read off on v. 

(2) With M.M.F. (^. e. A) and speed constant and the pole tips 
at exactly the normal distance apart, note the reading (A) on v. 

















































ELECTRICAL ENGINEERING TESTING 


213 


(3) Repeat 2 for eight different distances increasing by J at a 
time to the maximum possible. 

(4) Repeat 2 and 3 for a return set of distances to the minimum 
(normal). 

(5) Plot curves in each case with distances between iron of 
armature and pole face as abscissae and iV"as ordinates. 

y —(1) Adjust the excitation to maximum and the speed so 
that a suitable low reading of, say, maximum voltage is obtained 
on V. 

(2) With N (i. e. E) and speed constant and the pole tips at 
exactly the normal distance apart, note this distance (d) and the 
exciting current A. 

(3) Repeat 2 for eight values of (d) rising by ^ of the maximum 
at a time to the maximum, noting A, at each position, which is 
necessary to keep E constant. 

(4) Repeat 2 and 3 for a return set of distances to the minimum 
(normal). 

(5) Plot curves in each case with M.M.F. as abscissee and {d) 
as ordinates. 


Name . . . 


Date . . . 


No. Armature conductors C = . . External diam. iron core = . . . inches. 

Total F.M. turns (r)= .. . Internal „ „ „ 

Nett length „ „ =... „ 


Siieed in 
Eevs. 

Distance 
between 
pole t'ps 
(d). 

Distance 
between 
iron of 
face to 
core. 

Exciting 
Current, A amps. 

E.M.F. on 
(y)E. 

M.M.F. 

iir . _ 
= — AT 
10 

Flux 

N. 

108F. 
~ C.n. 

Per 

Min. 

Per 

Sec. 

(n). 

Increas¬ 

ing. 

Decreas¬ 

ing. 

Increas 

ing. 

Decreas¬ 

ing. 







' 

1 



Deductions. —State very clearly all the inferences which you 
can draw from your results and point out their bearing on 
dynamo design. 


(8o) Localization of Faults in Magnetizing 
Coils. (Induction-Ballistic Method.) 

Introduction.—When a magnetizing coil of insulated wire is 
wound on a metallic bobbin, the latter is usually insulated on the 
inside by a thin strata of insulating material before winding on 

























214 


ELECTRICAL ENGINEERING TESTING 


fehe covered wire. Notwithstanding this, it may and does some¬ 
times happen that the wire core becomes “ shorted ’’ to the metal¬ 
work of the bobbin, through the covering and insulation of the 
bobbin. This is particularly liable to be the case in shunt coils 
of dynamos which are wound on metal “formers,” insulated with 
vulcanized fibre tissue before winding. 


Such a fault, through poor contact of, in many cases, a very 
uncertain nature, gives trouble in the ordinary methods of testing 
for its position, by giving unsteady readings. Thus the ordinary 
resistance methods are extremely liable to be vitiated by variable 
contact resistance at the fault. The following method for local¬ 
izing the position of the faul^ by means of induced currents^ 
measured ballistically, is often a more convenient and reliable 
one for the purpose. 

Apparatus. —Metallic bobbin or former F to be tested, wound 
with the magnetizing coil (m) which is “ shorted ” to frame at the 

point (/) j high resist¬ 
ance ballistic galvan¬ 
ometer G', two-way key 
K (p, 587); battery 
of secondary cells B ; 
switch N; ammeter A, 
and temporary primary 
magnetizing coil PP 
wound over the outside 
of the magnetizing coil 
mm proper, which is to 

be tested ; rheostat R (p. 606); known high resistance box r. 

N.B.—It will be noticed that, as represented in Fig. 82, the 
fault {/) is on the first layer of turns next to the frame P'j and we 
will suppose that the turn at (f) is making contact there with 
the metallic frame (F). Thus it will be seen that the point (/) 
divides the total number of turns on the whole bobbin into two 
parts between the leading out wires of the coil, so that 

total turns = turns between T-^ and f + turns between and f. 

Observations. —(1) Connect up as in Fig. 82, and adjust A 
and G to zero, the temporary coil PP having been previously 
wound on and a wire soldered to any point (^ 9 ) on the metallic 
bobbin frame F. 



B — 


Fig. 82. 



















ELECTRICAL ENGINEERING TESTING 


215 


(2) With R full in, close S and adjust the current on A to 
some convenient amount. Next also close K to stud 1 and adjust 
r to such a value as will give, say, or f scale deflection on G 
when S is opened suddenly. Repeat two or three times with the 
same constant current, both made and broken in P. 

(3) Close K to stud 2 and repeat 2 above with the same cur¬ 
rent, noting the new resistance out in r to give a suitable first 
throw on G. 

(4) Repeat 2 or 3 for about four or five current strengths A so 
as to obtain finally different throws on G which will check one 
another, and calculate the position of the fault (/), or the number 
of turns to be unwound, to reach it, from the relation 

N-^ turns between and,/ mean 1st throw 
.A^g'^i'Urns between 1\ and /"mean 1st throw d^ appiox. 
where are the total resistances oi r + G when obtaining d^ and 
c ?2 respectively, and which are assumed to be very large compared 
with the contact resistance at f and also the resistance of the 
turns between / and both 2\ and T^. If the resistance of the 
coil [m) is from 5 to 20 ohms then (i'-\-G) should if possible be at 
least 10,000 ohms. 


Name . . . Date . . . 

Coil tested : Total Turns N— . . . Total Resistance 
Galvanometer Resistance (?= . . . Ohms at . . . ° C. 


Current 

for 

references 

only. 

1st throws on G. 

Box resistance. 

Circuit Resist. 

Ratio 

Ni/Na 

Turns to 
unwind 
Na. 

mean 

di. 

mean 

d^. 

r 

r> 

II 

r' + G 
= Gi). 










N.B.—It will be noticed from the formula in 4 that if r is 
adjusted so that d^ = d^^ then 

= - or -^2 = /fv 
^ ^2 ^ » 1 + ^2 

or again if r is kept constant throughout. 




then N^IN^ ^2 = ^ 

If G is insensitive an iron core may be inserted in the coil to 
form a closed circuit if possible; this will increase the flux for a 
given current made or broken in PP, and therefore also the first 
throws cZj d^o^ G. 




























216 


ELECTBIGAL ENGINEERING TESTING 


This has the further advantage that N-^ and N^ will now enclose 
the same number of lines of force, which is only approximately 
true if there is no iron core and the coil long. 

It should bo observed in passing that even a simpler method 
still than the one described above, for finding the position of the 
fault (/), would be to employ a slide wire or meter bridge or other 
convenient form of potential divider in the following manner. 
Connect the ends 7\ Fig. 82, of the faulty field coil to the ex¬ 
tremities of a meter bridge wire and also to two or three Leclanche 
cells; connect the galvanometer G, which need not now be bal¬ 
listic, but which must be sensitive, between tlie metallic former 
at p and the slider key of the bridge wire. Now move the key 
such that on tapping it G does not deflect. Then the lengths 
T-^f and fT^ of the faulty coil are in the proportion of the corre¬ 
sponding lengths of the stretched wire either side of the K, and 
are therefore known if the gauge of winding and its resistance 
(which can be measured in the ordinary way) are known. 


(8i) Determination of the Rise of Temperature 
and Increase in .Resistance of Magnet- 
windings. 

Introduction. —Since every magnet coil has some resistance, 
which is usually considerable in shunt or pressure coils but 
small in series or current coils, it follows from Joule’s law that 
heat must be generated in them when excited. The amount of 
heat developed per second by a current of (/) amperes flowing 
through, or a pressure of (F) volts across the terminals of, a coil 

p2 

of R ohms resistance is cc I'^R or Any coil must therefore 

have such an external surface for radiation of heat relatively 
to the amount of heat developed in it, that the “steady” 
temperature attained when the rates of production and dis¬ 
sipation of heat become equal is not high enough to deteriorate 
the insulation of the winding. The maximum limit to this 
“steady” final temperature is usually fixed at about 50° C., for 
it is found that the commoner insulating materials used generally 
begin to deteriorate with temperatures exceeding 60 to 70° C. 


ELECTRICAL ENGINEERING TESTING 


217 


Admiralty specifications, however, prescribe that after a six- 
hours’ run at full load, no accessible part of a machine may 
show a temperature of more than 70° F ( = 38°*8 C) above the 
surrounding air. This would seem unnecessarily low, but from 
remarks to follow may not actually be so. 

In the case of dynamos and motors the rise of temperature 
and its final steady value is required for the armature, series 
or shunt coils, commutator, bearings and frame. Further, it 
has been shown that the radiating facility of a surface in 
contact with iron is nearly twice as good as when it is exposed 
to air. 

Except in special measurements and research, when perhaps 
thermo-couples and their equivalents may be used, the tem¬ 
perature rise of coils while energized is always obtained either 
(1) by thermometer, the bulb of which is placed on the coil and 
covered wdth a pad of cotton w^ool, or (2) by resistance measure¬ 
ment, obtained from the readings of an ammeter in series with, 
or a voltmeter across, the coil and the application of Ohm’s law. 
This latter method is the one usually employed in a test room, 
is the most accurate of the two, and the quickest method of 
finding the “true mean rise” of temperature, especially with 
series coils. With shunt coils this resistance method can be 
effected by switching the supply off and then quickly measuring 
the resistance of the coil by the Wheatstone Bridge method. 
Usually the true mean rise of temperature by resistance tests is 
found to be at least 1‘4 to 1*6 times greater than the apparent 
mean rise by thermometer due to the temperature of the layers 
of winding increasing from the outer one to that situated about 
three-fourths of the thickness of coil from it, and then decreasing 
again to the inner layer next to the iron core. 

If Rc = the resistance of the coil cold, and that when hot, 

then Rh = Rci^ + — ^c)) approximately, 

1 X , 

' or -r^ = —;-morc accurately, 

Rc 1 + a'c 

where fc and fji = the temperature in deg. cent, of the coil, cold 
and hot respectively, and a = the temperature coefficient of the 
material which for copper = 0*00428 ohm per ohm per 1° C. 

= f X 0-00128 = 0-00238 per °F. 



218 


ELECTllIGAL ENGINEERING TESTING 


,*. the rise of temp. = — ic) = — — 77 ^ 

a.he 

— 233 r deg. cent. = 420 deg. Fahr. 

lie l^c 

If now T = final temp, rise above surrounding air, 

S = total heat radiating surface in □" (exclusive of 
end flanges and internal surface, if any), 

W = total watts wasted in the coil at full load 
= total 


then T oz W cjz ~ or 

o 


W 


where {K) = a heating constant depending on the depth of 
winding, amount of fanning by the armature, and whether the 
surrounding air is still or circulating, and may be taken as 75 
for the usual shape and size of field coils of dynamos and 
motors, especially of multipolar types, excepting when iron-clad. 


Hence 


ir 

T=75-^ deg. 


cent. 


and since for shunt bobbins IF = VIsh — l^Sh^Sh' 
for a prescribed temp, rise (T) we have 


max. shunt current Ish = 


/ TN. 

V 


amperes. 


Apparatus. —Magnet coil F (of, say, a dynamo) to be tested; 
ammeter {a) and voltmeter [v) each capable of dealing with the 
full-rated current and voltage for the coil; switch watch; 
small bulb thermometer and cotton wool; adjustable high re¬ 
sistance r for shunt coils, or lov/ resistance for series coils; 



ammeter voltmeter V, switch S, and adjustable load resistance 
for the main circuit to the armature d/. Separate means for 
driving Af. 

Observations. —( 1 ) [Ar7natm'‘e M stationary]. Connect up as 
shown on the left half of Fig. 83 and adjust a and v to zero, if 
necessary. Note the temperature of the air of the room by the 













ELEGTBIGAL ENGINEERING TESTING 


219 


thermometer, secure the thermometer with its bulb touching the 
outside of the coil, and cover the bulb with a pad of cotton wool. 

(2) AVith r full in, close and quickly adjust r so that v or 
a shows the normal value for the coil, and note the readings of 
both V and «, the thermometer and the time. 

(3) By adjusting r maintain [a) constant in testing a series 
coil, or (v) constant in testing a pressure coil, either at the above 
normal value, and note the readings of v, a, the thermometer 
and time, say every 10 minutes for the first 1| hours, and then 
every 15 or 20 minutes, up to the condition when the variable 
quantity becomes constant. Then, again, take the temperature of 
the room and tabulate as follows— 


Name . . . Date . . . 

Coil Tested Type . . . Thickness . . . External Surface S = . . . □" 

Temp, of Room at Start = . . . °C. at End of test = . . , 'C. 


Time 

of 

Reading. 

Minutes 

from 

start. 

Amps 

(a). 

Volts 

(v). 

Watts 

JF = a.v. 

Resist¬ 
ance of 
Coil 

a 

Calculated 
Temp. Rise 
n,, - R. 

T = 

Ther¬ 

mometer 

Reading 

t. 

Whether 
Motor 
at Rest 
or how 
Running. 





1 






(4) [^Armature II driven at Full Load and at Normal SpeeF \.— 
Repeat obs. 1-3 after the machine has cooled down to the 
temperature of the air. 

(5) Plot curves to the same axes having time in '■'■minutes 
from start” as abscisste with values of R,^, and t as ordin¬ 
ates; calculate the “heating constant” {K) from the relation 

TS 

K = —, and the maximum value of shunt current suitable for 

W 

coil tested for the value of ( K) found, and for a final temperature 
rise T oi 50® C. above air. 

Inferences.—Clearly state all that can be deduced from the 
results of the test, and point out their bearing on temperature 
testing. 

(82) Efficiency of Direct Current Dynamos. 
(Swinburne’s Electrical Method.) 

Introduction.—This method, due to Mr. James Swinburne, has 
the advantage, firstly, in point of accuracy, of being solely an 
electrical one, and therefore far more accurate than a dynamo- 























220 


ELECTRICAL ENGINEERING TESTING 


meter method in which the powder required to drive is measured 
mechanically; secondly, of not requiring another similar machine 
for coupling to it, in addition to the one tested. The method, 
which is often termed the “ Stray Power ” method, is consequently 
very suitable for employment in workshop determinations, where 
usually no good transmission dynamometer is available for 
measuring the H.P. used in driving the generator under test, 
and is invariably used when Hopkinson’s method cannot be 
applied as, e.g.^ when no second similar machine is available. 
The principle of the present and all similar methods is based on 
the following, namely, that the total power put in = total power 
given out total power lost internally or in symbols 

Wi = Wo + Wl 

where the suffixes /, 0 and L denote the input, output, and 
total losses in Watts {W) respectively. 

Thus the commercial efficiency {rj) of the dynamo is at once 
obtainable from the relation— 

Wo _ TFo 
’ w, fFo+JFj, 

The output in Watts developed by the dynamo is at once 
deducible from the product of the volts V and amperes 0 given 
out. The total loss JFz in Watts occurring internally in any 
dynamo is made up as follows— 

(a) Copper losses L^ in armature and exciting coils due to 
heating by the passage of current, and which can easily be calcu¬ 
lated when the currents and resistances are known. 

(b) Friction losses Lp due to air churning, journal and brush 
frictions. 

(c) Magnetic frictions or iron losses due to Eddy or 
Foucault currents and magnetic hysteresis. Hence the total 
internal loss IF^ = 4 + Xj, + and to the quantity {L^^L^ 
Mr. Swinburne has given the somewhat appropriate name of 
“ Stray Porrer” 

The copper losses are calculable as follows_ 

Let C = the current given by the dynamo at its normal voltage 
F to some external circuit, and let Rg^ R^,^ be the resist¬ 
ances of the armature series coils and shunt coils respectively of 
any dynamo to be tested, of which Rsj^ can be measured by a 
Wheatstone Bridge and R^ Rg, by the “ Potential Difference’’ 
method (p. 84). We shall then have for a 



ELECTRICAL ENGINEERING TESTING 


221 


Series dynamo Le= {Rai- Rse) 

F2 

Sliunt dynamo Lc ~ jr' 

11 


(-0 




Compound dynamo (long shunt) 

y2 




Sh 


+ 




-^Rst) 


Compound dynamo (short shunt) 


L. 


(c^ 


(V-Cll,.) 


It 


Sh 




R. 


Rsh 

The remaining losses, {. e. the stray power (Z^ + Z,„), can 
readily be obtained by running the dynamo as a motor, the 
field magnets being separately excited so that the armature has 
the same magnetic induction as at full load, the E.M.F. supplied 
to it being at least equal to the total E.M.F. which the machine 
would develop when running on full load as a dynamo at normal 
speed. Thus the machine is running on no load other than its 
own friction, eddy currents, and hysteresis. If ^ = current flow¬ 
ing through the armature and = the voltage across its terminals 
when the speed is up to normal, then we have 

Stray power = {Lp 4-Z,„) = AVa-La 
where = copper loss in the armature for the current A in it. 

Note.—Only a comparatively small current (A) at the proper 
E.M.F. mentioned above will be required to be furnished by the 
auxiliary source of current, and if R^ is very small. La can be 
neglected in comparison with A Fa in this last formula. 

Apparatus. —Dynamo 3/ to be tested, which for the purposes 
of discussion merely we will assume is shunt wound ; voltmeter 
V; low reading long scale am¬ 


meter A ; rheostats R (p. 606) 
and r (p. 599); tachometer ; 
complete Wheatstone Bridge 
set (TF.^); two-way voltmeter 
key E (p. 587); switch S.j; 
source of current E 2 A 2 ^ suffi¬ 
ciently high E.M.F. 

Observations.—(1) Connect 
up as in Fig. 84, and adjust 
the instruments Y and A to 
zero if necessary. Switch on E, 
when the field should be then 


ATMJV 

r 


Sh 


R 


■<^l 



26 


<- 

-0| 


tn 




Fig. 84. 



















222 


ELECTRICAL ENGINEERING TESTING 


excited to the normal amount, as can be seen by closing A 1, and 
observing whether the normal voltage which the machine would 
give as a dynamo at the proposed speed is indicated on V. 

(2) With R at its maximum value (not less than about 10 ohms) 
close S^, adjusting R so as to give the armature the full requisite 
E.M.F. E. It will still run under the normal speed, since with 
so small a current the armature produces no demagnetizing 
action to quicken it up. Now adjust r so as to bring it up to 
the normal speed, and note by closing K 2 the volts Va across the 
armature terminals and the current A amps, flowing through it. 

(3) Repeat 2 at the same excitation for some ten different 
speeds in all, both below and above normal (by varying 7?), 
enterino: the readings in the small tabular form—• 

O O 


Speed 

in 

r.p.m. 

Stray Power readings 

Total intake 
watts rr.nning 
light A Pa. 

Volts 

Va- 

Amps. 

A. 

Watts 

AVa-A»J?a. 







(4) Open E, and K and measure by means of AV.B. the 
resistance Ra of the armature and R^j^ of the shunt, remembering 
of course to disconnect one from the other while measuring their 
respective resistances. 

(5) Calculate the power supplied and the commercial or nett 
efliciency of the dynamo for some ten different values of 
currents C (at, say, constant speed and voltage) taken from the 
machine, ranging from 0 to full load by about equal increments, 
and tabulate as follows— 

Name . . . Date . 

Dynamo tested: No. . . , Normal Voltage = . . . Volts. 

Type . . ,, Current = . . . Amps 

Maker ... Speed = . . Revs, 

Resistance: Armature = . . . Ohms. @ °C. 


Series 


Normal Speed 
Revs, per min. 

Power developed 
assumed for 
calculation. 

stray Power 
Measurement in 
obs. 2. 

Losses. 

Calculated 
Power 
to Drive 

IPq + Rx 

Commer¬ 

cial 

Efficiency 

= 100^^%. 
Jij 

Volts P. 

CO 

P* 

Total 

Output 

Wo=yc. 

Volts Pa. 

Amps. A. 

Lp d" Lm 
= A Fa 
- A’^Ra 
Watts. 

Copper 

calcu¬ 

lated 

I.C. 

Total 
lFi = 

Lc + 

+ Lm. 


















































ELECTRICAL ENGINEERING TESTING 


223 


Note. There will be,only one value, that corresponding to 
normal speed, in each of the columns 1, 2, 5, 6, and 7 (counting 
from left to right) in the last table, but as many values in the 
remaining columns as there are values of amps. C assumed 
between 0 and full load. 

(6) Plot the following curves having — 

{a) Efficiency as ordinates and Watts developed as abscissje. 
{h) Stray power as ordinates and speed of armature as 
abscissjB. 

(c) W^atts developed as ordinates and W^atts to drive as 
abscissae. 

Inferences. —State clearly all that can be inferred from your 
experimental results. 

(83) Efficiency of Direct Current Generators. 
(Hopkinson’s Electrical Method.) 

Introduction. —The earlier methods of measuring the efficiency 
of direct current generators, in which the electrical output of the 
machine was obtained by the product of the ammeter and volt¬ 
meter readings, while the total mechanical input was obtained 
by means of some suitable form of transmission dynamometer, 
are more or less limited in their application from the fact that 
a reliable dynamometer is not always available. Even when it 
is, the method gives only an approximate result, for the error 
made in measuring the efficiency is proportional to the error 
made in measuring the input as given by the transmission dyna¬ 
mometer, and which is only too easy to make in an appliance such 
as this. It will therefore be evident that, given accurately cali¬ 
brated instruments, any method of measuring the efficiency solely 
electrically will be capable of giving far more accurate results 
than could be obtained with any dynamometer. 

The present method has this advantage, of being solely an 
electrical one, and requires two machines of as nearly the same 
output as possible, the accuracy of the test practically depending 
on how nearly alike in this respect the two machines are. 

They must be capable of being placed in alignment with their 
shafts coupled mechanically together. The test can be made 
with either series, shunt, or compound machines, but the shunt 
is much the simplest. 


224 


ELEGTRIGAL ENGINEERING TESTING 


Apparatus. —Accurate ammeters A and ; voltmeter V ; 

rheostats circuits (p. 599); change over 

voltmeter key K (Fig. 
254); dynamo (a) to 
be tested coupled both 
mechanically and elec- 
trically to a similar ma¬ 
chine (^) vdiich runs as 
a motor. An auxiliary 
source of current (y), 
such as a storage battery, 
or another dynamo giving 
an E.M.F. about equal 
to the normal of a and yS, 
and able to supply the 
losses occurring in the 
machines a and /?; switch 



S", rheostat Rh (p. 606, Fig. 274). 

Observations.—(1) Connect up as in Fig. 85, and make sure 
that the E.M.F. of y assists that of the dynamo a in driving the 
motor in the right direction for self-exciting a. 

(2) The respective fields Fg. and in series with rheostats 
R^ and R^ respectively, are excited from the terminals of y, as 
shown, to the normal amount roughly, except that of /?, which is 
weakened to enable it to run as a motor. 

(3) With Rh full in to start with, close S and adjust the 
auxiliary source of E.M.F. (y) and the rheostat {Rh) so that the 
machines get up speed, and if possible obtain the normal full 
load current of a through the circuit. 

(4) Slightly re-adjust R-^ and R<^ to bring a(I up to normal 
speed, then in quick succession measure the volts Fj at the 
terminals of the dynamo a and the volts F 25 at the motor by 
means of the key K, at the same time noting the main current 
on A and the exciting currents a-^ and 

(5) If possible obtain three or four different load currents 
through ay 8 from the normal downwards, and calculate the effici¬ 
ency % from the relation 


^ = 


^ approximately. 


and tabulate in a convenient manner. 








































J^LECTRIGAL ENGINEERING TESTING 


225 


Inferences. —Show how the above relation can bo obtained, 
and state any assumptions made in obtaining it. What correc¬ 
tions would have to be applied to make it rigorously true? 
Obtain the true value of the efficiency % by applying the correc¬ 
tion in question. 

The test, though simple, requires a certain amount of experi¬ 
mental skill, especially in the case of series and compound 
machines. Moreover, the starting is somewhat troublesome. 

By a sliglit modification in the connections, the test is a little 
easier to carry out, and this is shown in Fig. 86. Like the preced¬ 
ing arrangement it involves the use of an auxiliary generator or 



set of secondary cells having the same current capacity as the 
machines under test, and a voltage of from 8 to 25% of that of the 
generator, according to their efficiencies. This, being, as before, in 
series with the generator and motor, takes the form of an added 
voltage to the system. 

It is much better to excite the shunts from an independent 
supply instead of the auxiliary source. 

In this arrangement the motor /? must have the strongei field, 
and in order to start, the field Fa of the generator a must 
either be broken or be made comparatively weak by means of the 
rheostat 

Apparatus.—Similar to that for the preceding test; source of 
B.M.F. (^E) necessary to fully excite the shunts Ea and F/?, the 
Auxiliary Source y being as above mentioned. 

Observations.—(1) Connect up as shown in Fig. 86, and adjust 
the ammeters A and cq and voltmeter V to zero, etc. 


Q 



























226 


ELEGTBIGAL ENGINEERING TESTING 


(2) With 7?! and Rh full in and the voltage (^;) of the source 
E at the requisite value, close S, adjusting Rh to obtain full load 
current A through a and /?, then simultaneously take the read¬ 
ings of a, v, A and the volts V-^ and Fg across a and y by means 
of the key K. 

(3) Calculate the efficiency of either machine from the 
relation— 



and tabulate your results in a convenient manner. 

(84) Measurement of the “Nett” or “Com¬ 
mercial ” Efficiency of Direct Current 
Dynamos. (Kapp’s Electrical Method.) 

Introduction. —The following, being an electrical method en¬ 
tirely, has the advantage that all the measurements are electrical, 
thereby enabling the efficiency to be determined with far greater 
accuracy than would be possible with any mechanical transmission 
dynamometer. 

The method consists in coupling the generator to be tested 
both mechanically (with their armatures in alignment) and 
electrically to a similar type machine of as nearly equal power 
as possible, and which latter is made to run as a motor, driving 
the other, by the weakening of its field, with a rheostat. A 
small auxiliary generator, giving the normal voltage of the 
machine to be tested, is required, and must be so connected that 
it can be placed in quick succession across the terminals of the 
two coupled machines. The auxiliary source therefore supplies the 
necessary exciting currents together with the difference of the 
currents flowing in the two coupled machines. The test, though 
simple, requires a certain amount of experimental skill, especially 
in the case of series and compound machines. 

Apparatus.— Dynamo (a) to be tested, assumed to be a shunt 
wound machine and having its field coils across its terminals; 
another similar machine S to act as a motor and having a 

' O 

rheostat R^ in its field “ change over ” switch C (Fig. 253); 




ELECTRICAL ENGINEERING TESTING 


227 



main rheostat (p. 606); 
ammeter A ; voltmeter V ; 
switch /S'j and auxiliary 
source of E.M.F. (y),which 
may consist of the town 
mains (if the supply 
is continuous current), 
secondary battery, or 
small dynamo giving the 
normal E.M.F. of the gen¬ 
erator a to be tested. 

Observations. —(1 )Con- 
nect up as shown in Fig. 

87, and adjust the point¬ 
ers of A and V to zero, if necessary. Arrange the machines a 
and in alignment and couple their shafts together by a 
suitable coupling. 

(2) Turn the “ change-over ” switch C to a, and with R^ large 
close aS'j and gradually adjust and consequently the current 
until the machines start. Then when they are running at a 
constant speed, with V reading the normal voltage of a, note the 
ammeter reading A a. 

(3) Quickly “ change over ” C so as to place the auxiliary 
source y across (I and note the ammeter reading A^ for the same 
voltage V as before. 

(4) Repeat 2 and 3 for some four or five different speeds, cur¬ 
rent, and voltages, and calculate the efficiency from the relation— 

Combined efficiency of the two machines = ^ 

Commercial efficiency of either machine == . — 

\ Aa 


Tabulate your results as follows— 

Name . . . 


Generator tested: No. ... Type. 
Machine Coupled : No. 


Maher.. . Normal Volts . . 
.Normal ,, . . 


Date . 
Amps. 


Speed . 


Speed in 
Revs, per 
min. 

Voltage 

V. 




Currents in Amps. 

Efficiency of 

^a. 

^ 13 . 

Combination 

Generator tested 
or the other 

100 

















































228 ELECTRICAL ENGINEERING TESTING 

% 

Inferences. —Show how the expression for the efficiency can be 
obtained, and dilate on the advantages and disadvantages of the 
method. 

The preceding method can be slightly simplified by the 
following modifications. As in the above test, the following one 
involves the use of an auxiliary generator or set of secondary 
cells, having the same voltage as the machines under test and a 
current output of about 8 to 25% of that in the armature of a 
or /5. Being in parallel with the machines to be tested, it takes 
the form of an added current to the system at the same voltage 
as the combination under test. The present tests are more con¬ 
venient, generally speaking, and much simpler as regards starting 
than those of No. 83. Fig. 88 shows the connections, and the 
apparatus required is much the same as in the preceding method, 
except that the change-over switch 0, Fig. 87, is dispensed with. 



The fields and can be connected as shown in Fig. 88 
instead of as in Fig. 87 if preferred, and it will then be noticed 
that 87 and 88 are electrically the same when the change-over 
switch C is kept as shown, and an ammeter {a) inserted in one 
of the leads connected to it. The source of supply y, whether 
power mains or a third generator, must have a voltage at least 
equal to that of either a or /3. Further, the losses in a and /3 
are measured directly, and are small compared with the output 
of a and yd; hence a small percentage error made in measuring 
them will be very small compared with the output of a and J3, 
and wTll have but little effect on the resulting efficiencies. 
When two machines of the same size and type have to be 
tested, this method is almost always used in works for deter¬ 
mining their efficiency and heating on a full-load time test. 


















ELECTRICAL ENGINEERING TESTING 


229 


Observations— (1) Connect up as in Fig. 88 and set all the 
instruments to zero. 

(2) To start up, put fall in and cut out short circuit, 
so that the fields and are as nearly as possible of equal 
and maximum strengths. Then close .S' and slowly cut out R^, 
when the machines will start up as two similar motors in 
parallel on no load. The ammeter A will now read about half 
that of [a) because a will be taking about half the supply 
current. 

(3) Now weaken the field F^ of the machine [I by slowly 
increasing R^, which will cause it to run faster and act as a 
motor, driving a as a generator. The reading of A will simul¬ 
taneously fall, while that of {a) will remain nearly constant; 
and when A becomes zero, the voltage of a will have reached a 
value just balancing that of the supply y, and {a) will indicate 
the current required to run a and /3 together at 0 load. 

On still further increasing R^ the current through {A) will be 
reversed, indicating that a is now commencing to supply, instead 
of receive, current. 

Note.— For this reason A should be either of the moving soft 
iron needle type of instrument, or of the moving coil permanent 
magnet type connected in circuit through a reversing switch, 
otherwise a central zero moving coil type must be used. 

(4) Take a series of load currents, as indicated on (/I), dififering 
by about equal amounts between 0 and the full-load value for 
either machine by still further increasing 7?2—noting the read¬ 
ings of all the instruments and the speed at each load, F being 
constant at about normal voltage. 

Note.— This circulating current A between the machines a 
and J3 will increase with the difference between their field 
strengths; and the limit is reached when the combination of 
a large current in the motor armature, and its weak field and 
high speed, causes excessive sparking. 

Tabulate your results as follows— 


230 


ELECTRICAL ENGINEERING TESTING 


Name . . . Date . . . 

Machine a: No. . . . Type . . . Normal Output = .... Volts =* . . . 

Amps. = . . . Speed = . , . 

,, p : No. . . . Type . . . Normal Output = . . . Volts = . . . 

Amps. = . . . Speed = . 


Speed 

in 

r.p.ni. 

Generator (a). 

Current 
from 
Mains 
(a) Amps. 

Input into 
Motor /3 
(A + a)r. 

Efficiency of 

Amps. 

(A). 

Volts 

(H- 

Output in 
Watts 

W = A. V. 

Combination 

Ai - ^ . 

.11 J -h a 

Either 

Machine 







i 


(5) Plot curves having values of E and E-^ as ordinates, with 
W as abscissse. 

Inferences. —What errors, whether small or large, is the 
method liable to, and on what does the accuracy depend 1 


(85) Measurement of the Commercial Effici¬ 
ency of a Generator by means of a 
Transmission Dynamometer. 

Introduction. —This method of measuring the efficiency of an 
electrical generator, namely, by means of a transmission dynamo¬ 
meter, can be applied to a direct current generator equally as 
well as to an alternating current one. As, therefore, the 
application of the method to each of these two great classes of 
machines, to form two separate tests, was considered superfluous, 
preference was given to its application with an alternator, in 
that the output of a direct current generator is at once given 
by the product of the volts and amperes, while that of an 
alternator may present some difficulty to obtain accurately, the 
reasons for which are carefully explained. The actual measure¬ 
ment of the driving power by the dynamometer is obtained in 
precisely the same manner no matter what generator is being 
tested. 

There are many different methods of finding the commercial 
efficiency of an alternator, depending in some cases on whether 
the armature rotates or is stationary, on the capacity of the 
machine, and on the facilities at hand for testing. In all cases 
the commercial efficiency = “ mean ” useful power developed-r 
total power absorbed by the alternator, the latter being = power 





















ELECTRICAL ENGINEERING TESTING 


231 


applied to the pulley to turn it + the power used in exciting. 
The mean or true power developed is easily obtained if a non- 
inductive lesistance, such as a bank of glow lamps or water 
rheostat, is at hand which will carry the full load current of the 
machine, for then the true power = amperes x volts. This will 
not be true if the resistance is inductive owing to the “ phase 
difference” between the current and voltage. For such a case 
the true power may be obtained by a non-inductive Wattmeter 
or the o-voltmeter method (p. 379), etc. The power applied at 
the alternator pulley to drive it is very commonly obtained by 
indicating the engine, especially in large “ sets.” In the present 
case a transmission dynamometer is used to measure this power. 
It is of the spring type, and the means for recording the 
readings of it were devised by Prof. W. Stroud. The indications, 



which are recorded electrically, represent the nett pull, or 
difference of tensions in the two sides of the belt in lbs. Then 
knowing the speed of the alternator and the diameter of its 
pulley, the H.P. can at once be deduced. For a full and 
detailed description of the dynamometer, see Appendix, p. 625. 

Apparatus. —Alternator D to be tested; transmission dynamo¬ 
meter complete with its indicating galvanometer G (p. 625); 
tachometer j a-c ammeter {A) and voltmeter (F); D-G am> 
meter (a); switch S ; and non-inductive resistance or bank of 
lamps R (p. 598); exciting circuit containing ammeter (a), 
rheostat r (p. 599), switch and exciting E.M.F. E. 

Observations. —(1) Connect up as shown in Fig. 89, and see that 
all lubricators in use feed slowly. Adjust the secondary E.M.F. 
(p. 629) for use with the dynamometer, so that when placed 
directly across the terminals of G, a full scale deflection is 
produced. Then insert it in its proper place. 









232 


ELECmiCAL ENGINEERING TESTING 


(2) With the alternator belt on the loose pulley on the 
counter-shaft, start the motor which drives this shaft, and 
note the mean deflection on G for different speeds. If this 
is appreciable it must be deducted from each of the readings 
which follow. 

(3) Now throw the belt on to the fast pulley so as to start I), 
and without S being closed or the field excited, again note the 
mean deflection on G for different speeds. 

(4) Adjust the speed of I) to give normal frequency and the 
excitation to give normal voltage on V. Note the reading of G, 
with S still open. 

(5) Close S and repeat 4 (keeping the speed constant) for 
about ten different load currents on A, rising by = increments 
to the maximum permissible by varying (A). 

(6) Repeat 4 and 5 for frequencies of 40% and 75% of the 
normal respectively, and tabulate as follows— 


Name 


Date . . . 


Alternator—No. . .. Maker . . . Normal output = . . . Watts, at . . . Revs, per min. 
Resistance of exciting coils (r) = . . . ohms. Diameter of alternator pulley (^ = . . . ft. 

,, ,, armature (warm) ra = . . . ,, Circumference ,, „ mi = . . . ft. 


Speed Revs, 
per min. 

N. 

Frequency 

y*-s 

per sec. 

Deflection 
on G. 

Nett belt 
pull 

(T-t) lbs. 

Exciting 
Currents 
(a) amps. 

Total 11. P. absorbed 

{T-t)ndN a’^r 
33000 74G 

Output. 

Useful H.P. 
developed 

77 

~ 746 ■ 

Commercial 

efficiency 

= ioog%. 

Amps. 

A. 

Volts 

V. 












(7) Plot curves for each speed having A and useful H.P. 
'developed as abscisste, and V and efficiencies as ordinates 
respectively. Also between H.P. developed as ordinates, and 
H.P. required to drive as abscisste. 

Note. —The nett pull of the belt in lbs. must be obtained 
from the deflection of G with reference to the latest calibration 
curve of the dynamometer. 


The Testing of Continuous and Alternat¬ 
ing Current Electro-Motors. 

General Introduction. —Since the production of the electro¬ 
motor in its more practical form within recent years, the uses 

























ELECTRICAL ENGINEERING TESTING 


233 


to which it has been applied, for the electrical driving of work¬ 
shops, haulage, electric traction, etc., etc., have assumed such 
proportions as to make the different forms and types of electro¬ 
motor at the present day multitudinous. The systematic testing, 
therefore, of such machines becomes of considerable importance, 
in order that a comparison may be obtained and a judgment 
formed of the weak points of any particular type, together with 
its performance and qualities (whether good or bad) which it 
possesses. 

No motor, least of all one intended for electric tram and 
railway work, should leave the makers’ works or be installed in 
its proposed occupation wdthout being first thoroughly tested for 
the following points— (a) Resistance, or conductivity of its 
electrical circuits; {h) Insulation resistance between earth or 
framework of the machine and the copper circuits both indi¬ 
vidually and collectively; (c) Brahe horse-power; (d) Efficiency; 
(e) Healing, or rise of temperature of the various parts of the 
machine after a run at full load for a specified time. These tests 
we may now consider more in detail. 

(a) Copper Resistai^’Ce.— That of each of the copper circuits 
should be separately measured, by the Wheatstone Bridge in the 
ordinary way (p. 81) in the case of the shunt coils or other 
circuit of several ohms, and by the Potential Difference Method 
(p. 84) or voltmeter and ammeter method (p. 86) in the case of 
the armature and series coils or other low resistance. 

(/;) Insulation Resistance. —That of the various parts can be 
obtained byTestsNos.43 and 49 (pp.ll3,129) or other convenient 
method, at a pressure of something like three or four times the 
normal working pressure of the machine. The insulation resist¬ 
ance of the machine as a whole, when tested at the normal 
voltage, should nob be less than 2000 ohms per volt, whence, of 
course, that of the individual coils or circuits will be much 
higher. Some makers merely test the separate parts under a 
pressure of 2500 to 5000 volts alternating, and if they stand this 
they are passed as satisfactory. This pressure can conveniently 
be obtained by means of a small testing transformer, stepping 
up from, say, 100 to 5000 volts, carefully set fuses being placed 
in circuit to prevent damage should the insulation break down. 

(c) Brake Horse Power.—T his may be measured in one of 


234 


ELBGTEIGAL ENGINEERING TESTING 


three waye, depending on the facilities 
at hand for testing; namely, by an 
absorption dynamometer, in other words, 
a modified form of Prony brake, by the 
“balance” or “cradle” method, or 
lastly by the electrical method. The 
last two methods will be described in 
conjunction with their application to 
tests which follow later on, but we will 
now consider the principle involved in 
the first-named method, reserving the de¬ 
scription of some convenient forms of 
brakes until later. It will be sufiScient 
if W 0 consider the principle of the 
simplest form of brake, consisting of a rope or band ahc, of 
diameter or thickness (d), lapped with any arc of contact 0 (in 
circular measure), from a fraction of a turn to more than one 
turn, over the face of the motor pulley P, having a radius (r) 
and which rotates we will suppose counter-clockwise, as indi¬ 
cated in Fig. 90. To one end c is ai^tached a large weight 
IT, and to the other (a) a small one w. Now when the pulley 
P is at rest, jr= tension on the right-hand or “tight” side of 
the rope, while - 11 ; = the tension on the left-hand or “slacker” 
part of the rope. Then, as P rotates, the couple or torque 2\ 
due to the force of friction between the rope and surface of the 
pulley, tending to resist motion, and against which the motor 
does work, is— 

T— {IV-io){r -f Je?) pound feet, 

where {TV and w) are in lbs. and {r and d) in feet, {TV — w) 
being the difference in tensions or nett load on the brake in 
lbs., and {r-\-\d) the mean effective radius in feet (of pulley 
and rope together) at which the nett load acts. If (?^) = number 
of revolutions per minute made by P, then 27r7i-^ 60 = w, the 
angular velocity of the pulley, and the work per second, or the 
rate at which work is done by the motor on the pulley = o)7\ 
Hence we have— 

II.P. developed = ( IF -14?) (?•-^J^Z) 27^7^-^ 33,000, 
where 1 H.P. is equivalent to 33,000 foot-lbs. per minute. 

All the power thus measured and appearing at the pulley is 










ELECTRICAL ENGINEERING TESTING 


235 


wasted in heating this latter, and herein lies one of the chief 
difficulties in testing larger H.P.s, namely, the getting'rid of 
the heat so generated by friction, for not only is the heat liable 
to burn the rope in two if the power of the motor is sufficient, 
but it also alfects the co-efficient of friction (fx) between the 
rubbing surfaces, thereby causing the brake to jerk and prevent¬ 
ing any steady readings being taken. 

To obviate this trouble, either the pulley must be water-cooled 
(see p. 633), or readings must be taken immediately after adding 
a weight, and then the weight released from the rope. The 
trouble is further intensified by the motor running at such fast 
speeds, which is common to this type of driving power. By a 
slight modification of this form of brake, viz. substituting a 
spring balance for {w)j the brake becomes automatically self¬ 
regulating for variations of /x, for then if /x suddenly increases, 
W rises, and (iv), which now is the spring-balance reading, de¬ 
creases, therefore W-io increases and restores the brake to its 
first position. The coefficient of friction /x can be calculated 
thus— 

Let 0 = arc of contact (in circular measure) between cord and 


pulley, then — = 
w 




where € = base of the Napierian logarithms = 2-71828. 

The friction surfaces (in contact) of the brake should be as 
large as possible, in order to readily dissipate the heat generated. 
Mr. Maw gives the following rule for finding the smallest 
dimensions of a brake pulley: if H.P. = horse-power to be 
measured by the brake, and -y = peripheral velocity of the pulley 
in feet per minute, and (^ = width of rubbing surfaces in contact. 


measured axially, then ■ must not be less than 700. 

{( 1 ) Efficiency. —This can at once be obtained if the electrical 
H.P. absorbed by the motor for a given B.H.P. is known. If 
A amperes as read off on the ammeter is passing into the machine 

at a P.D. of V volts read on the voltmeter placed across the 

A V 

terminals of the machine, then the input or E.H.P. = 

where 1 H.P. = 746 Watts. -o tt t) 

. - _ . B.H.P. _ B.H.P. 

Hence the commercial efficiency ^ ^ p - — j^g-p 






236 


ELEGTRIGAL ENGINEERING TESTING 


{e) Heating. —This may be limited by specification or the 
question of safety to the conductors, and also considerations of 
overloading. It is not advisable that the rise of temperature of 
any part of the machine should exceed 40° C. above that of the 
external atmosphere after a six hours’ run on full load. The 
temperature can be obtained by placing the bulb of a thermometer 
on the part to be tested and covering it over by some cotton 
wool. This can only be done to the armature at the moment of 
stopping, and it will here be noticed that a sudden rise of surface 
temperature occurs in the armature at the moment of stopping, 
due, of course, to the ceasing of the ventilating action which 
goes on while it is rotating (see p. 216). 

(86) Variation of Speed with Voltage across 
the Armature of a D.C. Electro-Motor 
(at Constant Excitation). 

Introduction. —This is an important test, in that it will 
familiarize the student with the fundamental principles under¬ 
lying the regulation and control of motors. It can be carried 
out on a series, shunt or compound-wound motor, so long as the 
corresponding change in the connections and means for main¬ 
taining constant excitation are made. As, lioweA^er, the same 
result is obtained with each type of motor, we shall operate the 
test with the simplest type, viz. the shunt motor. 

Note.—In a series motor the field regulating resistance, at 
least equal in value to the resistance of the series coils, must be 
shunted across them; whereas in shunt and compound motors it 
is connected in series with the shunt coils, and has a resistance 
and current-carrying capacity at least equal to those of the 
shunt coils. 

Apparatus. —Shunt motor, of which M is the armature and 
F the field; main circuit variable rheostat A, ammeter and 
switch /S', each capable of dealing with the full-load current 
of M] voltmeter V and supply mains of voltage E each 

for the rated voltage of M; field rheostat (r) and low-reading 
ammeter (a); tachometer. 

Observations. —(1) Connect up as shown in Fig. 91 and adjust 
the pointers of F, «, and A to zero if necessary. 


ELECTRICAL ENGINEERING TESTING 


237 


(2) With (?') all out and R full m, close S and gradually cut 
R out to short circuit as 31 gains speed, then adjust (r) to get 
normal speed {n). Note the readings of V, A, and a, which 
last-named must now be kept constant throughout the test by 
varying (r).—(See that the lubrication of d/is working). 

(3) With the motor still running light as in (2) above, vary R 
so as to obtain some eight different speeds (y?) in about equal 
steps between 0 and the normal, and note the corresponding 
readings of V, A, and a {a being kept constant throughout). 

(4) Repeat (3) with the motor running at full load (if arrange¬ 
ments permit), and for the same value of constant field current 
(a) as before. 

Note.—The loading-up of motor can most conveniently be 
effected by means of an eddy-current brake or by taking any 
desired output from a coupled generator. 



(5) Repeat (3) and (4) with, say, half the previous excitation 
maintained constant and tabulate all your readings as follows—■ 

Name . . . Date . . . 

Motor No. ... Tjpe . . . Annatiire Res. r — . . . olims . . . 

Full Load:—B.H.P. = . . . Volts = . . . Amps. = . . . Exciting Amp?. = . . • 

R.p.m. = . . . 


Motor 
running 
Light or 
Loaded. 

Supply 
Volts (K) 
on 

Armature. 

Back 

E.M.F. 

V = V — A.r. 
of 

Armature. 

Armature 
Amps. (A). 

Constant 
Field 
Ami>s. 
used (a). 

Armature 
Speed in 
R.p.m. (n). 

Calculated 

Ratios. 

n 

T 

n 

V 

E 

A 











(6) Plot to the same pair of axes, curves having speed (ii) as 

v , 1 c 17 4 ^ fsuppUj voltage across 31^ Mf\ 

ordinates with values or V, A, and (—-i 

as abscisste. 

Inferences. —State clearly what you can deduce from the 
table of results and curves, and show how they can be applied 
































238 


ELECTRICAL ENGINEERING TESTING 


to the design of a main circuit current rheostat for controlling 
the speed of the motor. 


(87) Variation of Speed with Excitation in a 
Direct Current Electro-Motor (with Con¬ 
stant Supply Voltage on Armature). 

Introduction. —The reader should peruse the introduction of 
the last test, the remarks in which aj^ply to the present test 
also. In addition, it may be pointed out that when the motor 
is running light the back E.M.F. will remain nearly constant, 
since the power required to drive is usually very small, and is a 
(V-v), consequently the increase of speed will be almost inversely 
a to decrease of field strength. 

Apparatus. —That required for the present test is precisely as 
detailed for the last one, and need not be repeated again here. 

Observations. —(1) Connect up exactly as shown in Fig. 91 
of the last test, and adjust the pointers of V, a and A to zero if 
necessary. 

(2) With if) all out and Rfull in^ close S and gradually cut 
R out as the motor gains speed, until V reads the normal 
voltage of the motor; then adjust (r) to get normal speed. 
Note the readings of A, a and F, which last-named must now 
be kept constant throughout the test by varying R (see that the 
lubricating arrangements of the motor M (Fig. 91) are working). 

(3) With the motor still running light, as in (2) above, vary (r) 
so as to obtain some 8 speeds (??), differing by about equal steps 
between 0 and the normal value, and note the corresponding 
readings of A, a and V (F being kept constant throughout). 

(4) Repeat (3) with the motor running at full load (if arrange¬ 
ments permit), and for the same constant value of F as before. 

Note. —The most convenient way of loading up M (Fig. 91) is 
by means of an eddy current brake, or by taking the required 
output from a coupled generator. 

(5) Repeat (3) and (4) with, say, half the previously normal 
value of supply voltage F across the armature, maintained 
constant, and tabulate as follows— 


ELECTRICAL ENGINEERING TESTING 


239 


Name . . . Date . . . 

Motor: No. . . . Type . . . Armature Ecs. (7') = . . . ohms. 

Field Coil Res. ( 7 -) = . . . ohms. 

Full lead : B.II.P.= . . . Volts. = . . . Amps. = . . . Field Amps.= . . . R.p.m. = . . . 


Jfotor 
nuiiiiiig 
light or 
loaded. 

Armature 

Field Flux 
oc 1/a. 

Ela. 

Field Current ( 0 ). 

Supply 

Volts 

V (const.) 

Amps 

A. 

Speed 

(n). 

iVscending. 

Descenditig. 










(6) Plot to the same pair of axes, curves having speed (71) as 
ordinates with values of {A), (a) and (supply voltage ao'oss 
M 3 ^M 2 a) as abscissae, and between l/?i as ordinates with (a) 
as abscisste. 

Inferences.— State clearly what can be deduced from the table 
of results and curves, and show how these can be applied to the 
design of a field regulating rheostat for controlling the speed of 
the motor. 

(88) Variation of Voltage, Current, and Speed, 
with position of the Brushes around the 
Commutator of a D.C. Machine at Con¬ 
stant Excitation. 

Introduction. —Although the usual practice now is to design 
D.C. generators and motors with a fixed diameter of commuta¬ 
tion and immovable brush-bars, special cases are met with in 
which provision is made for moving the brushes through con¬ 
siderable angular space round the commutator. As is well 
known, the terminal voltage of a generator and the speed of a 
motor is each capable of variation by moving the brushes, while 
a motor can even be stopped and reversed by so doing. In fact, 
where the variation of voltage or speed required is not large, 
it can be obtained by brush movement without expensive field 
regulators and without altering therefore the field strength—a 
feature which is sometimes valuable, while most machines will 
admit of quite an appreciable brush movement without much 
sparking, when running light, such is not the case when they 
are running on load, so that the scope of this test may be limited 
by the amount of sparking. Further, it will be found easier to 





















240 


ELECTRICAL ENGINEERING TESTING 


apply the test to a motor than to a generator, and we shall 
therefore operate the present test on a motor. 

Apparatus.—Shunt motor, of which JI is the armature and F 
the field; main circuit variable rheostat {R); ammeter A, and 
switch S, each capable of dealing with the full-load current of 
M ; voltmeter V and supply mains of voltage E at least 



equal to the normal for J/; field rheostat r and low reading 
ammeter (a); tachometer and, if possible, some scale for indicating 
the angular motion of the brushes round the commutator. 

Observations. —(1) Connect up as in Fig. 92, adjusting (F) 
{A) and [a] to zero if necessary, and [R] to a value not less than 

, ^ . supply voltage ^ ^ ^ ^ 

the Ratio „ ^ ohms, so as to prevent the rull-load 

rull-load current ^ 

motor-current being exceeded if the armature comes to rest or 

its speed increasing too rapidly as the brushes are moved. 

Note.—This value of R will be given by blocking the armature 
and with normal excitation, noting the value necessary to give 
full-load current on A. 

(2) Start the motor either by using the ordinary “Starter,” 
or by {R) temporarily increased before closing S, and then 
gradually cutting out R until normal speed is reached— the 
brushes being in the normal full-load running position, and the 
excitation adjusted to normal full-load value. 

Now note the values of V, A and («), and the speed. 

(3) Next keeping («) constant at the above value, adjust R 
to the minimum value allowable and found in obs. (1), and note 
the readings of V, A and speed for normal position of brushes 
(as in obs. 2), and for a series of different positions throughout 
an angular distance == J the polar pitch either side of their 
normal position. 

(4) Repeat (3) at the nearest B.H.P. load to full load which 
it is practicable to run at, and tabulate as follows— 










ELECTRICAL ENGINEERING TESTING 


241 


Name . . . Date . . . 

Motor : No. . . . Type . . . 

Full load : B.II.P.= . . Amps.=! ' . , Volts = . . . E..p.m. = . . . 

Armature Resistance r„ = ohms. Normal Excitation = amps. 


Position of 
Brushes. 

Speed in revs, 
per min. (n). 

Armature 

Biek 

E.M.F (e) 
V-Ar„_ 

Field Strength 
oc e/)i. 

Volls V. 

Amps. A. 








(5) Plot curves on the same axes having values of brush 
position (normal as origin) as abscisste with values of e/?i, V, A 
and speed respectively as ordinates. 

Inferences. —State clearly what can be deduced from the 
results of your test. 


(89) Efficiency and B.H.P. of Direct Current 
Series Wound Electro-Motors. 

Introduction. —The series motor in general possesses some 
characteristic features which it may be well here to note in view 
of the prominent place this type of motor has, and still is, taking 
in electric traction and power work generally. Since it can be 
shown that the torque T of the motor is given by the relation— 
^_ CNAa _CN^ E-e_CN E-CNn 

27r 27r Va 27r 

where (7 = number of armature conductors all round, 

i\^=numher of lines threading the armature or the useful 
. flux, 

= number of amperes of current through armature, 
and e = impressed and back E.M.F.s of the mains and motor 
respectively, 

n = speed in revs, per second, 
and = resistance of armature circuit. 

It will be at once evident that the torque exerted is a maximum 
at starting, i. e. when n = o, and that it varies as the armature 
current Aa, since iValso varies as 

Again, when the motor is “ running light at its maximum 
speed T=0 nearly, for then the back E.M.F. generated almost 
=? that of the mains E. 

Thus a series motor tends to race directly the load is 

R 





















242 ELECTRICAL ENGINEERING TESTING 

suddenly removed, which is an undesirable feature for workshop 
driving. 

The fact that maximum at starting, and that the motor 
will start on full load, is a most valuable property for traction 
work on tram and railway lines. 

In the following Fig. and all after it, the motor is represented 
symbolically, (a) denoting the armature, commutator, and brushes, 
and EM the field magnet coils, which in this case, being series 
wound, are represented by a few curly lines. 

Apparatus. —Electromotor (series wound) to be tested, fitted 
with an absorption dynamometer or brake (Fig. 295) ; ammeter A ; 
voltmeter V; variable rheostat R (p. 606); switch S ; battery 
or dynamo R, giving the requisite voltage needed for the motor, 
and speed indicator and set of half-pound and one pound weights 
for the brake, also a lubricant if necessary. 

Observations. —(1) Connect up as indicated in Fig. 93, and 
adjust the pointers of A, V, and the tachometer to zero if 

necessary. See that all lu¬ 
bricating cups in use feed 
slowly and properly. 

(2) See that R is at its 
full, then carefully remove 
the brake from the pulley 
and close E. Take a series 
of gradually ascending a7id 
descending observations (by 
varying R) for about ten 
different speeds, ranging by 
about equal intervals between the lowest readable on the tacho¬ 
meter and the maximum safe speed for the motor, noting this 
speed and the corresponding values of A and V at each. 

(3) Replace the brake and repeat 2 for no weight in the 
pan. From 2 and 3 the loss in Watts in the brake can be 
found. 

Note. —It will probably be necessary to exert a very small 
pressure by the finger on the brake in 3 to prevent it being 
carried round as the pulley rotates. No appreciable error need 
be introduced due to this. If a form of brake is used in which 
no loss of power can occur other than that incidental to its 


Fm 



R 


Fig. 93. 
















BLECTRIGAL ENGINEERING TESTING 


243 


use when actually measuring power, then omit obs. 3 and also 
the last seven columns in the next table, and substitute a 
column headed A^V-^ watts to run motor at no load. 

Tabulate your results as follows— 


Speed 
Revs, 
per min. 
(n). 

Without Brake. 

With Brake. 

Loss in Brake 
in Watts 

R'ij = 

(A2V2-A1V1). 

Volts. 

Amperes. 

Volts. 

Amperes. 

Ascending. 

Descending. 

Qi 

D 

Ascending. 

Descending. 

rH 

(3 

c<3 

£> 

Ascending. 

Descending. 

c3 

<3^ 

Ascending. 

Descending. 

Mean A 2 . 
















(4) With the brake carefully replaced on the pulley and the 
smallest weight in the scale pan, close S and by varying R adjust 
the speed to the lowest convenient. Note this and the reading of 
V and A simultaneously, and the weight. 

(5) Repeat 4 at the same constant speed for ten or twelve loads 
or weights in the pan, ranging from the smallest to that which 
will cause the current to rise to not more than 25% over normal. 

(6) Repeat 4 and 5 for the maximum allowable speed and an 
intermediate one, each constant throughout, and tabulate your 
results as follows— 


Name . . . Date . . . 

Motor tested: No. . . . Type . . . Maker . . . Weight . . . 

Resistance: Armature r„ = . . . ohms. Series coils , ohms. 

Effective radius of Brake pulley and band r = . . . ft. 

Normal B.H.P. = . . . Amps. = . . . Volts = . . . Speed = . . . revs, per min. 


Speed 
Revs, 
per min. 
(n). 

Weight 
in Pan 
IF (lbs.) 

Lesser 
Weight or 
Spring 
balance 
reading 
w (lbs.) 

Nett 

Torque T = 
(JF-w) r 
(pound feet). 

Volts 

V3. 

Amj)s. 

As- 

Tota' 

absorbed 

746 ^ 

II.P. 

developed 
II1 = 

2 nnT B 

33000 

Commercial 

Efficiency 

=f'ioo% 










Note. —The true H.P. developed = 11.P. calculated + H.P. lost 
in brake itself. 

(7) With the brake removed from the pulley and E full in, 
close S and obtain the maximum speed allowable. Note this and 
also simultaneously the amperes {A^ and volts {V^. 

(8) Replace the brake and add weights to the pan so as to 
obtain about ten different loads to the point when the largest load 
























































244 


ELECTRICAL ENGINEERING TESTING 


stops the motor. Note the current and speed (n) at each, the 
voUs F 4 having been kejjt constant by altering R to suit the load. 

' Note. —The current should not exceed about 40% above the 
normal. Tabulate your results as above. 

(9) From observations 2 and 3 plot the following curves between 
{a) Yolts Fj as ordinates and the corresponding speeds (n) 
as abscissse. 

(h) Brake loss IFj, as ordinates and the corresponding speeds 
(ji) as abscissae. 

From observations 4-6, plot for each speed curves between 
(c) Efficiency and current as ordinates and corresponding 
B.FI.P.s as abscissae. 



From observations 7 and 8 plot the following curves between 
{d) The speed in each case and E.H.P. input, B.H.P., and 
efficiency. 

(e) The speed and current as ordinates and torque as abscissae. 

(10) Calculate the coefficient of friction (/x) between the brake 
band and pulley for various loads and for the arc of contact 
employed. 

From the curves 9 {d) deduce the relation between the speeds 
that give maximum efficiency and maximum B.H.P. respectively. 

Inferences. —State very clearly all the inferences deducible 
from your experimental observations. Explain fully the curves 
obtained in 9 id) above. 

Note. —The general form of the curves {d) 9 above are shown 





ELECTRICAL ENGINEERING TESTING 


245 


in Fi". 94. The diagram due to Mr. Kapp is an exceedingly 
useful one for seeing the relative H.P.s and efficiency. OcB is 
the B.H.P. curve, OaB is the efficiency curve, and Ahh is the 
E.H.P. (input) curve. The shape of this last varies with the 
type of series motor run off constant potential mains. The ordi- 

cd 

nates, such as {ad), of the efficiency curve OaB = ^ at this and 
similar points to any arbitrary scale of ordinates. 

* 

(90) Efficiency and B.H.P. of 500 Volt Direct 
Current Series-Wound Tramway Motors. 

Introduction. —The particularly heavy and trying work which 
a tramway or railway motor has to perform renders it all im¬ 
portant to subject the machine to the most searching tests for 
defects or other faults at the outset. Such tests are twofold— 

(1) A complete test of the motor at the works of the makers 
and again "when fixed to the car. 

(2) A test of its performance when driving the car on some 
approved route on the system. With regard to this case, the 
worst route of the whole system is chosen, ^. e. one having the 
steepest gradients and sharpest curves. 

The car is loaded with an artificial load, such as sand bags, for 
instance, equal to the full load of passengers which it is intended 
to carry. It is then run as continuously as possible along that 
route with five-seconds stops every five minutes for say two hours. 

This test is considered satisfactory, if, at the end of that time, 
all has gone on satisfactorily and the temperature of the arma¬ 
ture and commutator of the motor has not risen above the 
prescribed limit. 

Next, with regard to the “ Works Tests.” Besides the efficiency 
test at various loads, the motor should be run at the average 
speed it will run at in practice, say that corresponding to eight or 
nine miles per hour of the car, for four to six hours at the 
maximum load which the motor is intended for. 

Except in the case of electric railways, where the car or 
engine axle is direct driven, single reduction gear between motor 
and car axles is almost universally employed of between 4 75 and 
4‘86 to 1. 


246 


ELECTRICAL ENGINEERING TESTING 


The sizes of tramcar wheels are usually either 30 inches or 
33 inches in diameter. The remarks mentioned in the Introduction 
of Test 89 should be carefully read and remembered, when the 
performance of the motor on test will at once be obvious. 

Caution. — The oi^erators of the controlling rheostat switch-gear, 
etc., must stand on the india-ruhher mat iwovided, and must on no 
account touch any live metal work on the circuit of the 500 volt 
generator and tramway motor. 

Great care must he taken to insure that the rheostat in the main 
circuit of the motor is full in before closing the main switch, and also 
that it is at once re-inserted heiove pulling out that switch on stopping. 

The apparatus and connections of the preceding test are those 
now to be obtained, and the observations, as there given, to be 
carried out. In addition to curves 9, a — e, plot two on the same 
curve sheet, having speed as abscissae, and both torque and 
amperes as ordinates. 


(91) Relation between the Starting Torque 
and Current in a D.C. Electro-Motor, 

Introduction. —It is very instructive to compare the results 
obtained in applying the principle of the present test to series, 
shunt, and compound wound motors, but it may also be applied 
to alternating current motors. The torque or turning effort by 
the armature on its shaft, measured in terms of a pull, acting at 
a given radius or leverage from the shaft centre, and tending to 
turn it, is expressed in pound-feet, usually, in this country. In 
a motor it results from the inter-action of the field magnets 
and the field of the armature as set up by the currents flowing 
through it, and is a to the product of these two field strengths. 

If (7 = the number of effective conductors on the armature; 

N = the effective magnetic flux cut by them or flowing in 
the core; 

A = the armature current; 

then it can be shown that the torque {T) is given by the 
relation 

'f _ CN 
8-52 X 108 


X A. lb. ft. 



ELECTRICAL ENGINEERING TESTING 


247 


an expression independent of the speed of the motor. 

.‘.Ter, armature current X field flux a AN. 

In a series motor, the field flux N will vary as A varies, up 
to the point of magnetic saturation of the field magnets, v/hen 
it will be practically constant. Any further increase in A will 
give the relation 

T a A. 

This also holds for a shunt motor, in which the field excitation 
is practically constant and near saturation. 

In a compound or differential motor, however, the shunt and 
series windings oppose each other magnetically, and hence on 
starting it may happen that they nearly balance, thus giving 
practically zero starting torque. 



In capstan, windlass, and all traction work, maximum torque 
is required on starting; and since the series motor fulfils this 
condition and is exclusively used for the purpose in D.C. work, 
we will consider this type of motor to be the one used in the 
present test. 

Apparatus.—Series motor M to be tested on a suitable D.C. 
supply E ; brake-blocks B with yard-arm L and spring balance 
]V • ammeter A ; switch S ; and variable current rheostat R. 

Observations.—(1) Connect up as in Fig. 95, setting A and W 
to zero if necessary. Clamp B B to the pulley of the motor so as 
to prevent slipping (rotation), and with the yard-arm horizontal. 
Attach the spring balance W at a measured distance L from the 

pulley centre. 

(2) With Rfidl in, close S, and adjust R so as to give some 
ten currents through JI and A differing by about equal amounts 













248 


ELECTRICAL ENGINEERING TESTING 


between 0 and the full-load current of M, noting the readings of 
W and A at each. 

Note. —The connections of field F and armature of J/ must 
be such that the latter tends to turn in the direction shown. 
Also the yard-arm may have an equal overhang as shown, or 
be otherwise balanced when horizontal. 

Further, as there will be a good deal of static friction at the 
motor bearings, the armature should be rotated by hand a few 
revolutions before attaching B B, so as to well oil the journals; 
and even then the mean of several readings of IV, taken for each 
value of A by disturbing the position at which (Z>) rests, by the 
hand. 

(3) Take an ascending and descending series of value of A, 
and tabulate as follows— 


Motor: No. . . . T^pe . . . B.II.P. = . . . Volts. =. . , Amps. = . . . 


Amps. (A). 

Pull, ir (lbs.) 

Torque T = W,L. lb. ft. 

Values of AjT. 




1 


(4) Plot a curve having values of A as ordinates, and 1' as 
abscisste. 

Inferences. —Point out all that can be deduced from the table 
of results and shape of the curve. 


(92) Efficiency and B.H.P. of Direct Current 
Shunt-Wound Electro-Motors. 

Introduction. —If a shunt motor, supplied at constant poten¬ 
tial, has a very low armature resistance, high shunt coil resistance 
and field magnets giving a field relatively very much more power¬ 
ful than that due to the armature, the variation of “ lead ” of the 
brushes will be slight and the motor will be almost self-resfulatino- 
in speed for wide variations of load, i. e. it would run at constant 
speed independent of the torque. The falling off of the speed in 
shunt motors as the torque increases will be the less as the field 
magnetism is the more powerful. The brushes, in two pole 
machines, should press at opposite ends of a diameter, and to 
ensure sparkless running must have a “ backward lead.” In all 











ELECTRICAL ENGINEERING TESTING 


249 


cases the efficiency = total power given out -f- total power put in, 
both being reckoned in similar units. The input is easily deduced 
from ammeter and voltmeter readings, but the output is more 
difficult to obtain accurately. In the present test it is obtained 
by means of an “absorption dynamometer,” which we will assume 
to be the modified form of Prony Brake introduced by Paffard. 
Such a brake wastes in heat all the power given out by the motor 
through friction, but at the same time forms a measure of this 
power. The arrangement is such that the brake automatically 
adjusts itself to variations of the coefficient of friction between 
the rubbing surfaces due to heat. In brake tests of this nature 
just sufficient lubrication (such as soap and water) and no more 
ensures smooth working without sudden jerks due to seizing, and 
this, together with experience in manipulation, is the secret of 
the success of such tests. 

If (r) = mean effective radius of pulley and band together in ft., 
n = number of revs, per min., TF= weight in lbs. in scale pan, and 
w = weight in lbs. at the slack side of the pulley ; then the angular 
velocity of the pulley o) = 27m-^60, and the couple or torque 
resisting motion T={\V-w)r then the work done per sec. = ml', 
and the B.H.P. = (IF— w) 27rm-r 33000. 

Apparatus. —Shunt motor (d/) to be tested ; voltmeter V; 
ammeters A and a ; rheostats i?j (p. 606) and R 2 (P- ^99); 
switches and S .^source of current B ; speed indicator. A 
set of J lb. and 1 lb. weights are provided with the brake, 
together with a pump and tank by means of which a slight 
dripping of lubricant may be allowed to fall into the central 
rotating pulley and band. 

Note. —For further remarks on the testing of motors see the 
“ General Introduction” on the subject, p. 232, et seq. 

Observations. —(1) Connect up as indicated in Fig. 96 and 
adjust the pointers of all the instruments to zero where 
necessary. See that all lubricating cups in use feed slowly and 
properly. 

(2) Uncouple the absorption dynamometer from the motor 
shaft. Set Tij/iLg their maximum values, and close adjusting 
the exciting current {a) to the normal value by means of 

(3) Close aS'j and take a series of gradually increasing and 
decreasing observations (by varying R^ for about ten different 


250 


ELECTRICAL ENGINEERING TESTING 


speeds ranging by about equal intervals between the lowest 
readable on the tachometer and the maximum allowable for the 
motor in question, at constant normal excitation, noting the speed 
and corresponding values of A and V at each. 

(4) Repeat 2 and 3 for exciting currents (a) 50% below and 
20 % above normal respectively. 

(5) Re-couple the brake and motor together and repeat 2-4 
for no weight in the pan. From 2-5 the loss in Watts in the 
brake can be found. 

N.B.—It will probably be necessary to exert a very small 
pressure by the linger on the brake in 5 to prevent it being 



carried round as the pulley rotates. No appreciable error need 
be introduced due to this. Tabulate^ your results as folllows— 


Speed. 
Revs, 
per min. 
(n) 

Exciting 

current 

Amps. 

(a). 

Without Brake. 

With Brake. 

Loss in 
Brake in 
Watts 

Wb = 

AiVi) 

Volts. 

Amps. 

Volts. 

Amps. 

Ascending. 

Descending. 

a 

cS 

05 

Ascending. 

Descending. 

rd 

c 

<o 

Ascending. 

Descending. 

% 

O 

Ascending. 

Descending. 

o3 

<D 

















1 If a form of brake is used in which no loss of power oan occur other than that 
incidental to its use when measuring power, then omit obs. 5 and also the last 7 columns 
in the above table and substitute a column headed A-y Vi Watts to run motor at 0 load. 

( 6 ) Adjust both the exciting current (a) and the speed (n) to 
the normal for the motor being tested and keep both constant, then 
take a series of readings of .<4 3 and for some ten different loads 
varying from the smallest weight in the pan to the one which 
will give an armature current not exceeding 25% over normal. 

























































ELECTRICAL ENGINEERING TESTING 


251 


(7) Repeat 6 for a 50% smaller excitation at the same speed. 

(8) Repeat 6 and 7 for a 50% smaller speed. 

(9) For a constant voltage across the armature maintained by 
means of R^, load the brake with different weights, and note 
these and the corresponding speeds and currents through the 
armature at constant normal excitation, and tabulate as follows— 

Name . Date . . . 

Motor tested;: No. . . . Type . . . Maker . . Weight . 

Resistance—Shunt coils r, = , . . ohms. Armature = . . . ohms. 

Effective radius of brake pulley and band r = . , . ft. 

Current in Shunt coils . amps. 


Speed in 
Revs, 
per min. 
n. 

Weight 
in pan 

Jr(lbs.). 

Lesser 
weight 
w (lbs.). 

Nett 

Torque 

T~ 

r (IF-w) 
pound ft. 

Total 

H.P. developed 
27 rnT , 

Armature. 

Total H.P. 
absorbed 
i/2 = 

%efficiency 

=100x^ 

^2 

Amps. 

-^3- 

Volts 

^ 3 - 

33000 746 

746 











Note. —The true H.P. developed = H.P. calculated + H.P. lost 
in brake itself. 

(10) From experiments 2-5 plot curves between, volts Fj as 
ordinates, and speed (n) as abscissie, also with brake loss as 
ordinates and speed as abscissae. 

From experiments 6-8 plot curves for each speed, having 
efficiences as ordinates and total H.P. developed as abscissae, 
also between the latter and speed as ordinates. 

From experiment 9 plot the mechanical characteristic curve, 
having speed and current as ordinates and torque as abscissae. 

Inferences. —What can you deduce from the results of your 
experiments, especially from observation 9 1 


(93) Efficiency and B.H.P. of Direct Current 
Compound Wound Electro-Motors. 

Introduction.—The Compound Wound motor is an automatic¬ 
ally self-regulating one for maintaining constant speed independ¬ 
ently of the magnitude of the load. 

Without considering the theory of this regulation, which is 
outside the province of the present work, and for which the 
reader should refer to standard theoretical works, it may be 






















252 


ELECTRICAL ENGINEERING TESTING 


remarked that tlie desired result is obtained by employing the 
series and shunt coils to magnetize the field magnets differentially, 
i.e. while the shunt magnetizes, the series coils de-magnetize. 
This differential compounding results in the production of a nett 
field at any particular load sufficiently greater than what would 
be given by an equivalent pure shunt motor to cause the back 
E.M.F. to rise sufficiently to maintain the speed constant. The 
efficiency of such motors cannot manifestly be so high as one of 
the same size which is not wound in this way, since an extra 
amount of power is used up in producing the demagnetizing 
force which actually destroys part of the field. 

Apparatus. —Precisely similar to that required for the shunt 
motor test (p. 248). 

Observations. —These are the same as for the above-mentioned 
shunt motor test, and will not consequently be repeated here. 

The experimenter should refer to and carry the present test 
out in the same way, but in coupling up at the onset, care must 
be taken to connect so that the series coils oppose the shunt and 
tend to demagnetize the magnets. Exactly similar tabulation 
of results and plotting of curves must be carried out with the 
inferences deducible. 

N.B.—The applied E.M.F. to the motor should be maintained 
constant. 

(94) Efficiency and B.H.P. of Small Direct 
or Alternating Current Electro-Motors. 
(Cradle-Balance Method.) 

Introduction. —In testing small motors, such as from to ^ 
of a H.P., difficulties present themselves in measuring the power 
developed by them or the work which they will do, owing to the 
relatively large amounts of extraneous friction introduced in 
applying the usual brake tests. In fact, in the case of the smaller 
power motor, this source of friction would entirely vitiate the 
results and make them worthless. The following method prac¬ 
tically gets over this difficulty entirely, and may be carried out 
in one of two ways— 

(a) The motor to be tested is suspended freely with its arm¬ 
ature spindle in centres, or on friction wheels, the field magnet 


ELEGTRIGAL ENGINEERING TESTING 


253 


system with its bed-plate, etc., being carefully balanced by 
counterpoise weights so as to bring the centre of gravity of the ^ 
system in a line with the spindle. On the motor being supplied 
with electrical energy, and made to rotate and do work against 
the friction introduced at the face of its pulley by a stretched 
cord passing once round, the armature reacts on the field magnets 
tending to rotate them in the opposite direction with a certain 
force. 

If then this action is resisted by a weight or force W attached 
to the field magnet system at a leverage L, then the moment of 
this force resisting the tendency, i. e. the torque, = ^YL. 

Thus the arrangement is practically an electro-magnetic dyna¬ 
mometer in which the magnetic friction between armature and 
field magnets takes the place of mechanical friction in the ordinary 
dynamometer. 

The preceding arrangement of the method has the disadvantage 
that the weight of the heaviest portion of the machine is resting 
on the shaft, and consequently there will be a bearing friction 
assisting the magnetic pull of the armature on the field. 

A better arrangement in this respect is one devised by Prof. 
0. F. Bracket, which is merely a slight modification of the pre¬ 
ceding one. It consists in fixing the motor in a “ cradle ’’ sup¬ 
ported freely by knife edges resting on steel or agate planes, or 
on friction rollers, carried in a suitable fixed frame. The whole 
suspended system is very carefully balanced by means of counter¬ 
poise weights so that the centre of gravity lies in the axis of 
the motor shaft, this latter having been set in a line joining the 
knife edges of the cradle. 

A horizontal balanced lever controls the cradle, the end of it 
either supporting weights or being attached to a spring balance. 
Thus it will be seen that now the weight of the armature only 
is on the bearings, and it is being used under ordinary conditions. 
The balanced lever might be graduated and a sliding weight used 
to run along it to balance the torque, but the arrangement of 
the spring balance shown in Fig. 285 is the simplest and easiest 
to manipulate. This method has a further advantage that the 
friction at the journals of the motor does not affect or vitiate the 
measurement, but in the case of the application to a dynamo it 
should be remembered that it does. It should be borne in mind 


254 


ELECTRICAL ENGINEERING TESTING 


that a fruitful source of error may arise due to loss of power in 
driving the speed indicator. When small motors are being tested, 
care should be taken to choose an indicator that is very easily 
driven, and to drive it by means of a spiral or helical spring of, 
say, thin hard-drawn brass. Any eccentricity between the two 
shafts does not then matter so much as it would if they were 
direct coupled. 

Apparatus. —That required for this test is precisely similar to 
what is detailed under one or other of the preceding methods of 
testing series, shunt, or compound wound direct current motors 
or alternating current single phase motor, according to which of 
these types of motors the one being tested belongs. In addition 
the cradle absorption dynamometer is needed, for a complete de¬ 
scription of which see p. 621. 

Observations. —As, with the exception of the somewhat dif¬ 
ferent type of brake herein to be manipulated, the whole test 
will be precisely similar to one of the foregoing motor tests, de¬ 
pending on which kind of electromotor is to be tested, the 
rationale of this present test will not be repeated here. The 
experimenter should refer to the proper corresponding test and 
carry out the present one in an exactly similar manner, tabu¬ 
lating and plotting the results in just the same way. 

Though the following expression will bo found in connection 
with the description of the cradle dynamometer on p. 621, we 
may repeat that if W — weight or force applied at the end 
of the cradle lever in order to keep the same at zero when the 
motor is doing work, and if A = distance between its point of 
application and the fulcrum of the cradle, then the torque exerted 
by the motor = WL — T, and the work it does per sec. = mT 
= lirnT. 

Where n — speed in revs, per sec.— 


. •. H.P. developed = 


^rrnT 

550 


Consequently if different tensions are applied on the cord 
wrapped round the motor pulley, causing it to do various amounts 
of work, thereby taking in different currents (^) amps, at dif¬ 
ferent voltages F, the efficiency of the motor at each load is_ 

. H.P. developed ^ttuT x 746 

= H.P. absorbed = 55 0 x ^ K 





ELECTRICAL EKGINEERING TESTING 


255 


(95) Efficiency and B.H.P. of Direct Current 
Electro-Motors by Swinburne's Electrical 
Method. 


Introduction. —In the usual brake tests it is difficult and often 
impossible to obtain very accurate results, owing to variation of 
the co-efficient of friction between the rubbing surfaces and the 
resulting jerky behaviour of the brake. The advantage of any 
method, therefore, of measuring the input and output of a motor 
by solely electrical means will at once be apparent, as it is possible 
to obtain much more accurate results with such a method. 

The present method, which is purely an electrical one, is due 
to Mr. James Swinburne, and is sometimes termed the Stray 
Power ” method. The principle of it and all similar methods is 
based on the fact that 

Total Power given out = Total Power put in — Power lost 
internally, or in symbols, IVq — i - where the suffixes 0, J, 
and L denote the output, input, and total losses in Watts (IF) 
respectively. 

We thus at once obtain the commercial efficiency of the motor 

W, IK/ 

The input in Watts IF/ given to the motor is at once obtained 
by the product of the volts and amperes of the supply. The total 
loss TFj- in Watts we will consider now more in detail, and which 
in any machine is made up as follows: {a) the copper losses Lq in 
armature and exciting coils due to heating by the passage of 
current, and which can easily be calculated when the currents 
and resistances are known; (6) the friction losses Lp due to air 
churning, journal and brush friction ; (c) magnetic frictions or 
iron losses -L,n due to eddy or Foucault currents and magnetic 
hysteresis. 

Hence total internal loss TF/^ = Lc + + L„^, and to the 

sum {Lp + L„^ Mr. Swinburne has given the somewhat 
appropriate name of “ Stray Power.” The copper losses are 
calculable as follows — 

Let C = total current flowing into the motor from the supply, 
and let Ra , Rge, a,nd Rgh be the resistances of the armature, series 




25G 


ELECTRICAL ENGINEERING TESTING 


coils, and shunt coils respectively of any motor of which can 

be measured by a Wheatstone Bridge, and Ra^ ^se by the 

“Potential Difference” method (p. 84) or ammeter and voltmeter 

method, p. 86. Then we shall have for a 

Series motor Lq = [Ra + Rse )> 

72 / jr\2 

Shunt motor L(y = —— H ( - 77 - ) 

Rs/t \ Rsh/ 

working voltage, 

Compound motor (long shunt) 

fr. V 


Ra, where V = normal 


Ln = 


R 


+ - 


sh 


R 


sh^ 


Compound motor (short shunt) 

(V-CR,^)^ 


C^Rse + 


R. 


{Ra + ^^5e)) 


0 - R, 


sh 


R 


sh 


The remaining losses, i.e. the stray power {Lp + L,n), can 
readily be obtained by running the motor at no load, i, e. with no 
other load than its own friction, eddy currents and hysteresis, at 
normal excitation of the field. Then we have 


{Lp + Xm) = = stray Power, 


where A now = current flowing into the motor armature at 
voltage Va across the armature, and A'^r^is the copper loss in the 
armature occurring for this current and voltage. 


Note. —Only quite a small cmrent ai the normal voltage of the 
motor is required to be furnished by an auxiliary source of E.M.F., 
and if Ra is very small, A’^Va can be neglected in comparison with 
A Va in this last formula. 

Apparatus. —Motor 31 to be tested, which for purposes of 
discussion merely we will assume is shunt wound : voltmeter V : 


Sh 



low reading long scale amme¬ 
ter A; rheostat R (p. 606); 
tachometer; complete Wheat¬ 
stone Bridge set (W.B.) ; two- 
way voltmeter key K (p. 587); 
switch /S' 2 ; source of E.M.F. 
(E) at least equal to that for 
which the motor was built; 
rheostat r (p. 599) in the field 
coil circuit. 

Observations. —(1) Connect 
up as in Fig. 97, and adjust 
the instruments V and A to 






















ELECTRICAL ENGINEERING TESTING 


267 


zero if necessary. Insert E^ when the field should then be 
excited to the normal amount, which can be seen by closing K1 
and noting whether the normal voltage is read off on V. 

(2) With R at its maximum value (not less than about 10 ohms), 
close adjusting R and if necessary the excitation by the 
rheostat r, so that the machine runs at its normal speed. Now 
note, by closing K 2, the volts Va across the armature terminals 
and the current A amps, fiowing through it. 

(3) Repeat 2 at the same excitation for some ten different 
speeds in all,’both below and above normal, and tabulate as shown. 

(4) Open E, measure by suitable means the 

resistance Ra of the armature and Rg^ of the shunt, remembering 
of course to disconnect one from the other at the time. 

(5) Calculate the B.H.P. and commercial efficiency of the motor 
at normal voltage V for some ten different assumed values of 
current C supplied to the machine ranging from 0 to full load by 
about equal increments, and tabulate as shown in the larger table.^ 


Speed in 
R.p.m. 

Stray Power Readings (from obs. 2 and 3). 

Total Intake 
Watts Running 
Light A F„. 

Volts Va. 

Amps. A. 

Watts 







Name . . . Date . . . 

Motor tested : No. , . . Normal Voltage = . . . Volts. Resistance : Armature =s. . . Ohms. 

Type... „ Current =. . . Amps. Shunt = . . . ,, 

Maker . . . Speed = . . . Revs, per min. Series = . . . „ 


Nor¬ 

mal 

Speed 

Revs. 

per 

min. 

Power supply assum¬ 
ed for calculation. 

Stray PoAver Measure¬ 
ment in Obs. 2. 

Losses. 

Calcu¬ 
lated 
Output 
JVo = 
>rj-}Vz. 

Commercial 

Efficiency 

= 100 0/ 

Volts 

V. 

Amps. 

C. 

Total 
Input 
Wj = 

VC. 

Volts 

A.mps 

A. 

Watts. 

Coppei 

(calcu¬ 

lated) 

Lf. 

Total 
WF = 

(Lc+It 
+ LJ. 






1 


1 


(G) Plot the following curves having 

[а) Efficiency as ordinates and output Wq as abscissae. 

(б) Stray power as ,, and speed as ,, 

(c) Output Wo as „ and input IFj as „ 

Inferences.—State very clearly all that you can infer from the 
above experimental results. 

1 See note to larger table, p. 222-3, 












































253 


ELECTRICAL ENGINEERING TESTING 


(96) Efficiency of Direct Current Electro- 
Motors by Poole’s Electrical Method. 


Introduction.—This method,^ due to Mr. Cecil P. Poole, is an 
electrical one entirely, and enables the efficiency of an electro¬ 
motor to be obtained without using an absorption brake. The 
rated B.H.P. of tlie machine is assumed for the purposes of 
calculation, and the whole essence of the test consists in obtaining 
the armature current at which this rated output is obtained. 

Let A be this full load armature current. 

V = the normal voltage which the motor should have. 
Tr=the normal rated B.H.P. of motor, reckoned in Watts. 
V and a = the measured quantities as detailed below. 

Then the armature core + friction losses w = {V-v)a Watts, 

V . 

and the armature resistance r =— ohms. 

a 


Hence A — 


V - 


JV'^ — -f w) r 
2 r 



Tnis value for A is based on the assumption that the core losses, 
armature friction, windage, and eddy currents in the pole pieces 
all remain constant from 0 to full load. While this is not strictly 
the case, the error introduced is practically negligible. 

Apparatus. —Suitable source of supply of slightly higher voltage 
than the normal required for the motor to be tested; rheostat 
p. 599 ; low-reading long scale ammeter (a) ; voltmeter V to read 
the normal voltage ; low-reading long-scale voltmeter (v); and if 
the motor is shunt or compound wound, an ammeter to measure 
the normal shunt current ctsh > switch. 

H.B.—If the resistance of the shunt Tgh be known, then the 


V 

last-named ammeter may be dispensed with for ag^ — - —amperes, 

'i'sh 

Observations.—(1) Connect up the above apparatus so thajt 
the rheostat and ammeter (a) are in series with the armature 
alone, and the switch, so that it cuts off the supply entirely from 
motor and all apparatus, the voltmeter V being across the 
armature terminals, the shunt coils of the motor being across the 
mains. 


1 The rationale of the method first appeared iu the American Electrician, 
to which the author is indebted for it. 







ELECTRICAL ENGINEERING TESTING 


259 


(2) With the rheostat fulh in, close the main switch when the 
shunt will at once be fully excited. Now gradually cut out 
resistance in the armature circuit, thereby running up the speed, 
until V reads the normal voltage across the armature, thus running 
light. Now note the small armature current {ti) amperes flowing, 

(3) Switch off the shunt circuit and block the armature to 
prevent it moving. With the rheostat full in, close the main 
switch and again pass the same current {a), as in observation (2) 
above, through the armature wdiile stationary, noting the corre¬ 
sponding fall of potential {y) volts across the armature terminals 
by means of the low-reading voltmeter, and switch off. 

(4) Kepeat observation (2 and 3) twice or three times and take 
the mean of the respective values of (a) and (v), calculating the 
efficiency ^ of the motor on full load from the relation—- 

_100J^ 

F {A + a.k) 

and tabulate your results as follows—• 


Name . . . Date . . . 

Motor Tested: No. . . . Maker . . . Rated B.H.P = . . . 

Normal Voltage = . . . Speed = . . . Currenl - . . . 

Resistances : Armature = . . . Shunt = . . . Series = . . . 


Armature 
Current run¬ 
ning free, 
(a) ami)s. 

Volts across 
Armature 
Still 
(y). 

Losses 
(V-v) a 
= to 

Armature 

Resistance 

V 

r = — 
a 

Field 

Current 

ash 

Full load 
Armature 
Current A 

Efficiency 
of Motor 

2 









Inferences. —State clearly any advantages or disadvantages 
w’hich you consider the method possesses. 

















260 


ELECTRICAL ENGINEERING TESTING 


General Considerations Relative to the Test¬ 
ing of Asynchronous Alternating Current 
Induction Motors. 

While it is not proposed to discuss either the construction 
or the theory of action of such machines, certain considerations 
relative to the testing of both single and polyphase induction 
motors may with advantage be noted. In all cases they are 
self-starting by reason of the rotating magnetic field set up by 
the supply current, whether single or polyphase, flowing in the 
windings of the stator or fixed portion of the motor. It is, 
liowcver, only in single-phase types that after reaching full 
speed the rotating field (produced only during the starting- 
up period) is changed by switching to a simple alternating, or 
reversing, or pulsating field. 

The speed attained at the end of the starting period with 
no pulley load is called “full” or “synchronous” speed, but in 
all induction motors the speed of the rotor decreases as the 
load increases. 

If (/) = the periodicity of the supply in cycles per sec., 

(??,) = the speed of the rotor in revs, per sec., 

(= the number of pairs of poles in the stator, 
then synchronous or full speed is the speed of the rotating 

f 

field = — revs, per sec., while the difference between the speeds 

of the field and rotor, called the “ slip £ — n =£ -^ revs. 

P P 

per sec., and ~ ^ X 100 = 100 = the slip 

in percentage of full speed, which varies from about two or three 
in large motors to as much as twelve in very small ones. We 
therefore see that the slip equals the periodicity of the rotor 
currents. 

Measurement of Slip.—The last-named fact is made use of 
in the following method of measuring slip, but is applicable only 
in the case of induction motors with slip-ring rotors. Connect 
preferably a moving-coil permanent magnet D.C. ammeter in one 





ELECTRICAL ENGINEERING TESTING ' 


261 


of the leads between rotor and starter, tlien since such an 
instrument indicates for currents in one direction only, the 
number of impulses given to, or kicks (/i) of, the pointer per 
min. in the same direction will directly equal the number of 
complete cycles per min. of the induced slow period rotor 
currents—in other words the slip. If (/) = periodicity of the 

supply to the stator, then the percentage slip = 


K 


60 X / 
120 


100 . 


Thus, if /i = 120 kicks per min., the slip = x 50 ^ 


= 4% with a 50'^ per sec. supply. 

If a dead-beat moving soft iron needle A.C. ammeter is 
used, the number of kicks per min. would be doubled, for the 
same value of f and slip, and would, even if the ammeter 
was sufficiently dead beat to indicate with such rapidity, be 
impossible to count. With even a very dead-beat moving 
coil D.C. ammeter, a 5 or 6% slip is about the maximum 
measurable by this method. A slight variation of the above 
method consists in counting the oscillations of a light pivoted 
compass needle placed above or below one of the leads between 
rotor and starter, the lead having a direction N. and S. so that 
the needle lies parallel to it when no current is flowing. The 
slip is then obtainable as before. 

The above are direct methods of measuring slip, but if a long- 
range accurate tachometer is available, the slip can be obtained 
usually with sufficient accuracy by reading the rotor speed (??j) 
running light, and (^ 2 ) at any load when the slip is given by 

”1 ^ X 100%. 

Determination of Slip by Calculation. —If an induction motor 
has a three-phase wound rotor and both the rotor current 
{A^) and resistance (R^) per phase in each case are known, 
the slip {S) in cycles per sec., or in percentage of the supply 
frequency (/j), or in revs, per min. or per sec., can be calculated 
for the corresponding load as follows—• 

If y2 = frequency of the rotor currents, 

JFg = mechanical output from the rotor of the motor (in 
watts), 

p = number of pairs of stator poles. 





262 


ELECTRICAL ENGINEERING TESTING 


IFj = power (in watts) transmitted electro-magnetically by 
the rotating field in the stator to the rotor, 
tv = power (in watts) lost in the rotor = 
approximately. 

Then torque X = 11^2 

9 -f 

and torque X = 11^2 "h cos ^ for a 3-phase 

stator supply) 

. /, _ TF, TT^2 + ^ 

*• }\ 1^2 

and by a well-known rule in proportion we therefore have 

fl -f2 ff'l - lA ^ ^ 

where — 1 ^ 2=10 

. A ~ f2 ^^^2^2 

• • ® 'P +«) ~ 1^2 + 


Determination of Frequency, Slip, and 
Speed (Stroboscopic Method). 

Introduction. —Although the measurement of such quantities 
as those mentioned above—by this method—is by no means 
common, it probably only needs the advantages and accuracy 
of the method to be realized in order to bring it into much 
more general use. 

Measurement of Frequency and Slip. —The phenomenon and 
principles of stroboscopy can be applied in the measurement of 
either the frequency of an alternating current supply from an 
alternator, or the slip of an induction motor, as follows : A black 
disc having white radial lines is fixed concentrically on the shaft 
of an A.C. motor run from the supply, and is illuminated by an 
A.C. arc lamp fed from the same supply. Now the illumination 
from the lamp will vary periodically and flicker with the 
supply frequency, and when the speed of the stroboscopic disc 
corresponds with this supply frequency, i.e. when the angular 
velocities of the two are equal, the white lines will always be 
illuminated in the same place and appear to be at rest. If the 










ELECTRICAL ENGINEERING TESTING 


263 


speed of the disc is greater than that corresponding to the 
frequency of supply, the white lines will appear to slowly rotate 
in the same direction as the disc; whereas if the speed has a 
smaller value, the lines will appear to rotate in the opposite 
direction. The last condition will obtain with an induction motor, 
and if the number of white lines equals the number of pairs of 
stator poles, they will rotate in the opposite direction to that 
of the disc with the same number of revolutions per min, as 
those lost by the motor, i.e. as the slips. 

For example: the rotating field in a 2-pole stator on a 50 
supply will make one revolution in the periodic time of the current, 
or will rotate with a speed of 50 X 60 = 3000 revs, per min. If 
the slip between rotor and field is 5%(= 5 x 30 = 150 revs, 
per min.), a single white line on the black disc will appear to 
rotate backwards at a speed of 150 revs, per min., and will 
also make one complete revolution in the periodic time of the 
current. 

With a 4-pole motor and the same slip and supply frequency, 
the speed of the rotating field equals 1500 revs, per min., slip 
equals 75 revs, per min., and each of the two white lines will 
appear to rotate at 75 revs, per min., which can be counted 
against time, and the slip thereby at once obtained. Sectors, 
alternately white and black, can be painted on the disc or even 
the pulley, and used instead of the black disc with radial white 
lines if so desired. 

Measurement of the Resistance of Single and Polyphase 
Windings. —This is usually effected by the amine ter-voltmeter 
method with direct current (see p. 86) applied to a single phase¬ 
winding in the case of a single-phase generator or motor, and to 
each of the phase-windings separately of 2-phase machines. 

Thus, if (r) equals resistance of each phase-winding, we see that 
the total copper loss in a single phase machine equals A and in 
a 2-phase machine equals A -f A ; or if in the latter case 
the resistances of the two windings are equal as they should be, 
and usually are, we have r, and ii A^ = A^ then the 

total copper loss TTc = A^ X 2?*, where A is the current in 
either phase, and (2?*) the so-called equivalent resistance of the 
machine. 

In 3-phase windings, the resistance between any two terminals 


264 


ELEGTRIGAL ENGINEERING TESTING 


is, with star connection, that of 2 phase-windings in series (as 
seen from Fig. 143 a), and therefore = 2r; while with mesh 
connection (Fig. 143 h) we see that between any two terminals 
there are two circuits in ijarallel^ composed of 1 phase-winding 
in parallel with the other 2 phase-windings in series, or (r) in 
parallel with (2r), i.e. a terminal resistance of 


1 1 
1 , 1 =2-f 1 = 
r 2r 2r 



Now, if without troubling to trace the connections in order 
to see whether they are star or mesh, the resistance between 
the three pairs of phase-terminals are measured, and added 
together, the sum -f- 2 will equal the equivalent resistance of 
the whole stator or rotor windings, and the total copper loss 
in the stator or rotor = (line-current)^ x equivalent resistance. 


(97) No-Load ‘‘Open Circuit” Test of an 
Induction Motor on a varying Voltage, 
constant Normal Frequency Supply. 
(Rotor running Light at No Load.) 

Introduction. —Under these conditions the motor will run at 
its maximum possible speed, namely that corresponding almost, 
but not quite, to true synchronism, and therefore with an almost 
zero slip—the small difference being necessary for overcoming 
the small losses due to windage, mechanical and magnetic fric¬ 
tions, and copper loss due to the no-load running current. The 
test can be operated, of course, on single-, two-, or three-phase 
motors, but we shall assume the use of a three-phase motor here 
on account of the connections being slightly more complex. 

Such a motor may have either a squirrel-cage (short circuited) 
rotor or a wound rotor with slip rings. If the former, it may 
be started up from full voltage mains either by a star-delta 
switch or through an auto-transformer or sectioned choker, 
depending on its size. If it has a wound rotor, the starting 
rheostat connected to this is put to “full in” and the stator 
then switched directly to the supply. The starter is then 
gradually cut out to short circuit as the speed increases. If 





ELECTRICAL ENGINEERING TESTING 


265 


now, with the motor running at full speed, normal voltage, and 
frequency, the voltage is gradually decreased, the speed will 
remain practically constant until the lower voltages are reached, 
when it will fall off rapidly. 

Both the stator and rotor current will also decrease gradually, 
the former owing to a decrease in magnetizing and core-loss 
current producing the stator flux and depending on the voltage, 
the latter in an inverse proportion to the strength of the rotating 
field and voltasfe. 

O w 

As the voltage falls the idle or magnetizing component of the 
current will also decrease, while the energy component over¬ 
coming frictions will remain much the same in value, hence 



the ratio of idle to energy current will decrease and the powTr 
factor will in consequence rise. 

If Kg = normal voltage per phase on the stator — 
and Vji = the corresponding maximum voltage per phase of the 
rotor indicated when this is turned through a polar pitch with 


slip rings open-circuited. Then ~ is called the ratio of trans- 

' jt 

formation, which is not equal to the ratio of the stator and 
rotor currents owing to the magnetizing current of the stator. 
The induced E.M.F. in the rotor circuits varies directly as the 
slip, and has a frequency = the slip. Thus the reactance of 
the rotor circuits (= 27rZjj/j.) bears a constant ratio to the slip. 

As the rotor current is not usually measured, the ratio of 
transformation enables it, and also the most suitable starting 
resistance, to be calculated, knowing the stator current at any 
load. 

Apparatus.— Source E of three-phase alternating current, 
preferably a motor-driven alternator, the speed and field of 


























2G6 


ELECTEIGAL ENGINEERING TESTING 


which are each variable between wide limits; three-phase 
switch ; voltmeters Vs Kjt; ammetei'S AsAjj; frequency meter 
/; wattmeter IV ; reversing key (p. 585); and two-way key 
112 (p. 587) for fine-wire circuit of JV; three-phase induction motor, 
oE which (S) is the stator and (R) the rotor and (r) the starter. 

Note. —By the use of only one wattmeter, with its pressure- 
coil connected through /fg the remaining two mains in 
rapid succession, we assume the motor to be electro-magnetically 
balanced, or equally loaded, in the three phases (see p. 389). 
On no load, such is usually the case, for the current As is small, 
and therefore any inequality in the ampere turns, resistance, or 
reactance of the windings is nearly negligible in effect compared 
with what it would be on load. With phase-windings unequally 
loaded or balanced, two wattmeters must be used to obtain the 
true power absorbed (see p, 392). 

The ammeter Aju in the rotor circuit should be very dead beat 
and of the moving needle type, and should be of low resistance 
so as not to throw out the balance of the rotor currents. If the 
rotor li is of the squirrel-cage type, VmAj, and r cannot be used. 

Observations. —(1) Connect up as in Fig. 98, levelling and 
adjusting to zero, such instruments as need it. On starting any 
machine always see that its oiling arrangements are working 
properly before doing anything else. 

(2) With R running light at no load, adjust the voltage Vs 
and frequency / of the supply E to the normal values for the 
motor, and note the readings of all the instruments under this 
supply condition, and also (with the same frequency kept con¬ 
stant) for a series of values of Vs (by field regulation) decreasing 
by about equal amounts to the point where the speed begins to 
decrease, and from this point by smaller and more gradual decre¬ 
ments of Vg until the speed decreases too rapidly to read. 

Note, —After the speed begins to fall, sudden changes in Vs 
must be avoided, while the simultaneous readings of all the 
instruments must be taken rapidly. 

W must be read, first with its volt coil across (ah) and then 
with it across (he) at each voltage F^, by using the key Wg. 
Further, in some cases (not all), this change of connection will 
be accompanied by a reversal of direction of the deflection of IF 
depending on the magnitude of the power factor. The re- 


ELEGTRIGAL ENGINEERING TESTING 


£67 


versing-key JC^ must in such cases bo turned through 90°, so 
as to bring the deflection on to the scale again; but the reading 
must now be considered —and subtracted from the other to 
give the total watts (see p. 392), otherwise when the readings 
across ab and be are both on the scale and therefore both 
their sum gives the total watts. 

(3) With the frequency (/) constant at normal value for the 
motor, raise the voltage Fg from 0 by small and very gradual 
steps until the speed begins to increase too rapidly to enable 
readings to be taken. The simultaneous reading of all instru¬ 
ments at each voltage must be done rapidly. Tabulate all your 
results as follows— 


Induction Motor: No. . . . Maker . . . Type . . . 

Full-lead output: B.H.P. = . . . @ . r.p.ra. Volts = . . . Amps . . . Frequency . . . 
Equivalent Res. {hot) oi Stator Windings = . . . ohms: of Rotor W.ndings = . , . ohms. 


Speed by 

Supply 

Apparent Watts 

I 

§ II 4? 

g s> 

0 

Rotor 

. 

0 

rt *— ^ 

i| 

Value of 

1 

Tachometer. 

Kicks of 

Frequency 

(constant) 

I 

CQ 

'o 

Amps 

Watts. 

w 

Amps 


ft 

-H 

Total W = 
















(4) Plot the following curves (from obs. 2 and 3) having 
values of stator volts Vs as abscissae with (1) speed, (2) intake 

F 

watts TF, (3) stator amps. As, (4) cos (5) ratio -jf- as ordinates 

' li 


in each case. 

Inferences. —From a careful study of the shapes and disposi¬ 
tions of the curves relatively to the axes and of the tabular 
results, state all that can be deduced. 


(98) No-Load “Short-circuit” Test of an 
Induction Motor on a varying Voltage, 
constant Normal Frequency Supply. 
(Rotor kept stationary and short-cir¬ 
cuited.) 

Introduction.—Under these conditions the slip will be 100% 
since the speed is zero, while the power absorbed will almost 




































268 


ELECTRICAL ENGINEERING TESTING 


'svholly consist of copper loss cc to the square of the current. 
The only remaining source of loss is that due to hysteresis and 
eddy currents in the iron which will be small, owing to the low 
induction density reached with even the maximum voltage it 
is possible to use in this test. Further, it will be noticed, from 
a reference to test No. 137, that the motor approximates to a 
static transformer with a stator primary and rotor secondary 
under the conditions for maximum magnetic leakage which the 
stator windings maintain in the air gap between stator and 
rotor. 

Under stationary conditions the ratio of stator to rotor 
current is practically a constant and approximates to the ratio 
of transformation of the motor. Under running conditions the 
ratio of currents departs from constancy, due to the no-load 
current taken by the motor at normal voltage, and no longer 
approximates to the ratio of transformation. 

Apparatus.—That indicated for the preceding test (p, 265), 
but without an additional wattmeter now being used 

with its current coil in main a or c (Fig. 98), one end of each 
of the volt circuits of the two wattmeters, to be denoted by to-^ and 
W 2 ') being connected to the third main as indicated in Fig. 143c. 
The reason for now using two wattmeters is that the heavier 
stator currents to be used in this test, which will depend mostly 
on the resistances of the windings, will show up any slight want 
of symmetry, and may (or may not) result in the unequal 
current loading of the three phases—a condition necessitating 
the use of two wattmeters (p. 392). A^ Aji and 1^2 must 
also now be capable of taking the heavier currents used in the 
present test. 

Observations.—(1) Connect up as in Fig. 98, levelling and 
adjusting to zero such instruments as need it. See that the 
lubricating arrangements of the supply set are working 
properly. 

(2) With the rofor R shori-drcidted and prevented from rotating 
and the supply-frequency (/) constant at normal value, take the 
readings of all the instruments at each of a series of supply 
voltages Vs; increasing from zero to a value which will produce 
a stator current As of, say, 50 % in excess of that of full load, 
and tabulate as for the last test (p. 267). 


BLECmiCAL ENGINEEEING TESTING 


269 


(3) Plot curves, having values of Vg as abscisste, with values 
of ir, ^ 5 , Aji and cos ^ respectively, as ordinates. Also curves 
between As as ordinates, with Aj^ as abscissae, for this test, and 
from the readings of the last test No. 97 on the same curve- 
sheet for compaidson. 

Inferences. —From a careful study of the curves state clearly 
all that can be deduced from the test. 

(99) Efficiency and B.H.P. of Single Phase 
Alternating Current Electro-Motors. 

Introduction. —The somewhat rapid development of the distri¬ 
bution of electrical energy by single phase alternating currents 
in recent years has brought with it the introduction of single 
phase alternating current motors, of which, up to comparatively 
recently, there has been no practical commercial instance. Now, 
however, there are several forms, but none of them are able to • 
compete with the direct current motor in the matter of efficiency 
and powers of starting under load with the amount of electrical 
power absorbed in doing so. There are two classes of alternating 
single-phase motors, known as the Synchronous and Asynchronous 
types. The former cannot start themselves but have to be run 
up, by a separate source of power, into synchronism with the 
periodicity of the supply current; then, on being switched into 
circuit, they run perfectly synchronously with the generators, 
i. e. at constant speed, for wide variations of load from 0 to 
considerably over full load, and are of course separately excited. 
The latter class are self-exciting and self-starting (on very small 
loads) by using suitable means, but are non-synchronous, and the 
difference between the speeds of rotation of the magnetic field and 
the rotating armature is called the “ Magnetic Slip ” or “ Slij) ” 
simply. This generally only amounts to a small percentage at 
full load. 

The self-starting property is obtained by producing a rotatory 
magnetic field at starting, caused by diphasing the current in two 
separate circuits by means of the suitable use of either self- 
induction or capacity, one circuit being cut out when the motor 
gets up speed. 

The fixed portion of the motor (i. e. field magnets) through the 


270 


ELECTRICAL ENGINEERING TESTING 


winding of which tho supply current flows is usually termed the 
Stato?'.” The rotating portion (i. e. the armature) is termed the 
Rotor and usually consists of short-circuited conductors carried 
on a well-laminated drum. There is no electric connection in 
most cases to the rotor, or between rotor and stater. It will also 
often be found that the best efficiency is not at normal full load, 
which is analogous to the series wound direct current motor in 
this respect. Speaking broadly, it may be said that single-phase 
motors should be self-starting, and this on a current certainly not 
exceeding that taken at full load. The 2 yower factor should be high. 

A motor built for a given periodicity will not give as a rule 
its full power when supplied with a current of a much higher 
periodicity, while it will take too much current with a lower 
periodicity. 

The efficiency of any motor = the total power given out -j- the 
total ‘‘mean power” absorbed, both being reckoned in equivalent 
units. In the present and similar tests the true mean input 
cannot be obtained by the product of the amperes and volts, as in 
the case of direct current motors, owing to the “ phase difference” 
between the current and pressure, but must be obtained by 
means of a non-inductive Wattmeter. The output, or B.H.P., is 
obtained by an absorption dynamometer, which is a modified form 
of Prony brake. Such brakes waste, in heat, all the power 
developed by the motor from friction, but at the same time give 
a measure of this power. No lubricant is usually needed, but a 
little black lead may be applied to the pulley if the brake is jerky. 

Apparatus. —Alternating current motor M to be tested; brake 
complete with weights; non-inductive Wattmeter W; alternat¬ 
ing current ammeter A and voltmeter V ; switch S ; rheostat R 
(p. 597); tachometer; source of alternatiog current E, prefer¬ 
ably one that can be varied. 

Tests.—(1) Connect up as indicated, and adjust the pointers 
of all the instruments to zero, lev^elling such as need it. See 
that all lubricators in use feed slowly, and that the resistance 
switch (/S') is open. 

(2) Adjust the speed and excitation of the alternator so as to 
give the normal voltage and frequency required for d/, and remove 
the brake. 

(3) Make R a maximum; i. e. in the present case put resistance 


ELECTRICAL ENGINEERING TESTING . 271 


r-MAAAA 

R 



s 


fVW\J 


\AAAAA/ 


Fig. 99. 


switch S to start, and when the motor has got up speed throw S 
over from start to full, 
then, when the speed has 
become steady, note it and 

the readings of A, V, _ 

and W. 

(4) Place the brake in 
position, and with no 
weight in the pan, again 
note the motor speed and 
readings of V, and W. 

(5) Pepeat 4 for about ten loads, rising by about equal 
increments of w^eight to the maximum, the voltage and frequency 
being kept constant. 

(6) Repeat 4 and 5 for a higher and lower frequency than the 
normal. 

(7) Determine the power required to just start 31 by removing 
the brake, turning aS to start, and adjusting the speed of the 
alternator to give normal frequency (to be kept constant). 

(8) Carefully and gradually raise the voltage (at constant speed) 
by means of the excitation until 31 just starts, then instantly 
note the readings of A, V, and W. Repeat this three or four 
times and take the mean. 

(9) Repeat 7 and 8 for about five different frequencies, rising 
by about = increments to about 20% above normal. 

(10) Determine the relation between the speed of 31 and 
frequency of supply by removing the brake, and when the motor 
has got up speed, turning the Switch (S) to ‘‘full.” Then for 
constant normal voltage note the speed of 31 and readings of 
A, V, and W for about ten different frequencies, rising up to 
about 20% above normal. 

(11) Determine the effect of variation of voltage at constant 
normal periodicity with the motor running light by altering R, 
and noting the speed and readings of all the instruments. 
Tabulate all your results as shown. 
















272 


ELECTRICAL ENGINEERINO TESTING 


Name . . . Date . . . 

Motor: No.. . . T3 pa . .. Made by . .. Eflective Diam. of Pulley and baud d = ft. 
Normal B.H.P. = . . . at. .. volts and . . . revs. i)cr min., and frequency = ... per sec. 
Alterations per revolution of Dynamo K = .. . 


Speed in 
revs, per 
min. of 

Frequency 
per sec. 

60 

Volts V. 

CO 

s 

< 

Power Absorbed. 

Weight in Pan 

W lbs. 

Lesser weight or 

balance reading 

w lbs. 

Nett Weight in 

Brake (IF-w)lbs. 

B.H.P. developed 

traN^W-w) 

0 

0 

0 

CO 

CO 

I 

w 

H 

-i 

Power Factor 

cos.0= 

AV. 

Angle 9 . 

>> a 

0 

a 0 
<u 0 

0 

S X 

e^l rl 

1 Slip. 

Alternator 

n. 

Motor 

N. 

Apparent 

Watts 

AV. 

True 

Watts IT. 

a> li 

H H 


















Note. —The nett weight on brake = (weight of scale pan and 
weights) — reading of spring balance. 

(12) In experiments 1-6 plot curves having values of {a) 
power factor, {h) eflBciency, (c) true power absorbed respectively 
as ordinates, and B.H.P. developed as abscisste, for each 
frequency. 

(13) In experiments 7-10 plot curves having {d) true power 
required to start, (e) speed of motor, as ordinates and frequency 
as abscissae, in each case. 

Inferences. —What can you deduce from your experimental 
results 1 Taking the cost of electrical energy for power purposes 
at 2cl. per B.T.U., find the cost per B.H.P. per hour, and also 
when M is running on no load at normal voltage and frequency. 

(iod) Efficiency-Load Test of a Polyphase 
Induction Motor. (Absorption - Brake 

Method.) 

Introduction. —The efficiency of any electro motor 

B.H.P. developed B.H.P. 

E.H.P. absorbed B.H.P. -}- internal losses* 

The internal losses in an induction motor comprise (1) copper 
losses in stator and rotor windings, (2) iron losses (hysteresis 
and eddy currents) in stator and rotor cores, (3) mechanical 
friction due to windage, journals, and brushes, if it possesses 
a slip-ring rotor. 

If Ais and = the current and resistance, respectively,' per 
phase of stator winding and Aji and rji = the current and 









































ELECTRICAL ENGINEERING TESTING 


273 


resistance, respectively, per phase of rotor winding; then, if 

this latter is of a three-phase tyj^c, the stator copper losses 
are—- 

for single-phase; ^A^Vg for two-phase; and ^A\rs for 
three-phase induction motors, while the rotor copper loss is 

For a given iron core, we have seen (p. 354) that the expres¬ 
sion for the iron losses contains two variables only, namely, the 
frequency and the induction density oc to flux, and dependent 
solely on the supply voltage and number of stator turns. Hence 
the iron loss is independent of load. Further, since the fre¬ 
quency of the rotor currents and consequently of the flux in 
the rotor core equals the slip, which is only some 5 % of the 
speed of the stator field, it follows that the iron loss in the rotor 
core will be small compared with that in the stator core and 
the other losses, and will increase slightly with speed. The 
friction losses being oc to speed, will be sensibly constant at 
all loads in an induction motor, since the speed of such a motor 
has a variation of some 5 % only. It will therefore be at once 
realized that the copper losses (increasing as the square of the 
current) are mostly responsible for the rapid increase of the total 
internal loss as the load increases. 

The supply current to the stator of an induction motor is 
composed of two components— 

(a) One which may be termed the no-load or magnetizing 
component, producing the rotating magnetic field, and which is 
not only in quadrature with the supply voltage but nearly 
constant at all loads. 

(Owing to the air-gap between stator and rdtor-cores, the 
ampere turns of excitation, and hence the magnetizing com¬ 
ponent necessary to produce a given flux, is much higher, and 
the power factor much lower, than if the magnetic circuit was 
a closed one, and therefore an induction motor takes a consider¬ 
able no-load current which may be from a quarter to one-third 
of full-load current. The smaller the air-gap the smaller this 
current, the greater the power factor and output of the motor 
for a given size. For this reason the air-gap of such motors is 
reduced to a mere clearance for rotation.) 

(^) The other, which may be called the load-component, out 

T 


274 


ELECTRICAL ENGINEERING TESTING 


of phase with the voltage, but producing a field in the stator 
equal and opposite to that produced by the rotor currents in 
the stator, and hence balancing the demagnetizing effect of the 
rotor-induced currents on the stator field. 

This load component increases directly with the B.H.P. output 

„ , 1 i turns on rotor 

or the motor and — An X -7-7—• 

turns on stator 

Thus it will be seen that for the rotating field to have a 
constant strength, the stator current taken at no load will just 
suffice to produce this requisite field-strength and provide for 
the iron and friction losses. As the load increases, the increase 
in the rotor ampere terms is balanced by an equal and opposite 
increase in stator ampere turns, and we have the following 
relation, viz. that the 

Total stator amp. turns 

= total rotor amp. turns no-load amp. turns, 
or Total stator current 


= (rotor current ratio of transformation) -f- no-load current, 
the line above denoting that the sum is vectorial and not 
algebraical. 

The total power given out, i.e. the can be measured 

either by means of an absorption dynamometer brake in the 
manner already clearly defined in the previous tests, or by 
making the motor to be tested drive a direct current dynamo, 
the commercial efficiency of which is accurately known at various 
loads. This method should be adopted whenever possible, as it 
has the advantage, when carried out properly, of being more 
accurate than the ordinary brake methods. The method consists 
in suitably driving the dynamo from the motor to be tested either 
by means of a thin supple (pliable) belt or by the direct coupling 
of their shafts (placed accurately in alignment), through a flexible 
coupling or helical spring sufficiently strong for the purpose, 
thus avoiding the difficulty of getting their shafts exactly in true 
alignment. The belt arrangement also obviates the same diffi¬ 
culty, but it must be very pliable, otherwise errors will be intro¬ 
duced due to the extra power absorbed by the slipping and 
bending of this belt round the pulleys. 

Thus measuring the electrical power developed by the dynamo 
which is at once given by the current X voltage, and knowing 
its commercial efficiency e, the B.H.P. of the motor can easily 





i:LECTRICAt ENGINEERING TESTING 


275 


be calculated and = Further, if n = the speed 

in revs, per min., the torque T of the motor is given by the 
relation T B-H.P. x 33000 

27 ^?^ 

In all cases the efficiency of any motor = total power given out 
at its pulley ~ total power absorbed, both being reckoned in 
equivalent units. A multiphase alternating current motor is 
self-exciting, self-starting, but asynchronous as regards speed and 
the periodicity of the supply. The starting torque can be made 
equal to that of the best direct current motor without an 
excessive percentage over load in the current taken. 

They can be wound to run direct on 5000 volt circuits and 



over without much fear of the insulation breaking down, and 
their great advantage, except in the larger sizes, lies in the fact 
that there are no rubbing contacts of any kind to get out of 
order, and consequently there is no sparking. In the present 
case we will assume that the motor to be tested is of the three- 
phase type, as perhaps the measurements of input are not so 
obvious as in the two-phase system. 

Apparatus. —Source of three-phase alternating current E) 
three-phase motor SR to be tested either coupled mechanically 
to a direct current dynamo D of known commercial efficiency, or 
fitted with a Prony brake, p. 634; Wattmeter W ; alternating 
current ammeters Aji and voltmeters Vji; triple pole 
switch Sj^ S 2 Sg; tachometer; and if a coupled dynamo load is 
used, direct current ammeter A and voltmeter F; rheostat R 
(p. 606); switch S. 

Note. —For a detailed description of power measurements in 
multiphase circuits, see pp. 388-400. 

Observations. —(1) Connect up as in Fig. 100, and adjust all 
the instruments to zero, levelling such as require it. See that 

























276 


ELECTRICAL ENGINEERING TESTING 


all lubricating arrangements in use feed properly on starting 
the motor in the usual way. 

(2) With the motor quite free, take readings on all the instru¬ 
ments concerned when 31 thus runs “light” at its normal 
frequency and voltage, noting the speed. 

(3) If a dynamo load is used stop SR, couple the shafts of D 
and SR together and start the combination up again, with R at its 
maximum when S is closed; or if a Prony brake is used take a 
series of about ten different loads from D or on the brake, varying 
from the smallest to the largest permissible corresponding to the 
maximum current allowed for SR. Note simultaneously the 
readings of all the instruments at each load and also the speed, 
the supply voltage and frequency being constant throughout. 

(4) Repeat 3 for speeds 20% above and 20% below normal 
respectively, if possible, by varying the speed of the generator. 

(5) Adjust the direct current load to a convenient amount, 
then, keeping Vs constant, alter the speed of the three-phase 
generator by successive steps, and note the corresponding effect 
on W 2 and the speed of the motor. 

(6) Keeping the speed of the three-phase generator constant, 

alter Vs by successive steps and note the effect on and 

the speed of the motor, using the same load. Tabulate all your 
results as follows— 


Name . . . 


Date . . . 


Multipliase Motor: No. . . . Type . . . Maker . . . 

Normal: Volts = . . . Amiis. == . . . Speed = . , 

Res. Warm per Phase Winding : Stator = . . . ohms. 

Rotor (r^) = . . . ohms. 

Dynamo 2): No . . . Type . . . Maker , . 

Normal: Volts = . . . Amps. = . . . Speed = . . 

Diameter of Brake Pulley d = . . . ft. if used. 


Speed 

Revs. 

per 

min. 


o 

d 

C 

"q. 

a. 

0 

m 


<Kl 

cc 

p. 


Pi 

p. 

I 




o 

!> 


ce 

-p 

o 


Watts. 




For Use with Dynamo 
Load. 


lx 

QO 

4 ^ 

• 4 ^ 

d 


(D 

vB 

5 

a 

o 

a 


o 

O 

p; 11 

td tq 

W 


For Use with a 
Prony Brake. 



0) 

2 


02 

QQ 

n Tight 
IF lbs. 

P! . 

U 00 

04 X2 

0 ^ 

0 

0 

•—f 


2 

p 


Ph 




to 

^ -P 
AiC 

I ' 


P U 
c ■— 

O 1 

ta 

pq K 


® 

lx 
c IP 

CO 

-k 

cS ~ 

W 1 




O 

o 

^ § 
P II 

•g 

E 

a 































































ELEGTEIGAL ENGINEERING TESTING 


277 


(7) Plot the following curves from observations 3 and 4 for 
each speed having efficiency, power factor, slip, speed, current, 
and intake Watts as ordinates and. B.H.P, as abscissae, also 
curves having Torque as abscissae with A^, and slip as 
ordinates. 

And from observations 5 and 6, curves between voltage 
and supply frequency as ordinates with speed of the motor as 
abscissae in each case. 

Inferences.. —State very clearly all the inferences which can 
be drawn from your experimental results, and point out their 
bearing on electrical driving by multiphase current motors. 


(loi) Determination of the performance of 
an Induction Motor at all loads without 
loading it at all. (Heyland's Method.) 

Introduction. —It sometimes happens to be inconvenient and 
even impossible to brake, or otherwise absorb the B.H.P. of 
large induction motors, and in other cases to supply the large 
amount of electrical power required by them at full load from 
a generating plant which may be already running nearly at full 
load. The difficulty is met fortunately in such cases by the 
following method entailing the construction of the well-known 
“ Heyland Diagram ” from very simple “ no load and short 
circuit ” readings on the motor. The intake current and power, 
the output, the power factor, and the slip, etc., and hence 
efficiency, can then be deduced for all B.H.P.s and a complete 
set of curves drawn showing the performance of the motor. 
The temperature rise at any load cannot however be obtained 
with this method, and the best and most economical way of 
determining it is to let the motor drive a generator which is 
capable of absorbing the required B.H.P. and simultaneously 
return the output of this generator to the motor supply. Thus 
in running a 6 hours’ temperature test on, say, a 500 B.H.P. 
motor having an efficiency of 95%, the power wasted would only 


278 


ELECTRICAL ENGINEERING TESTING 


be some 10% or about 50 E.H.P. as against over 500 H.P. if 
the output of the generator had been taken up in rheostats. 

Apparatus. —Three-phase induction motor 31 with phases 
equally balanced (presumably) complete with starter. A tacho¬ 
meter j a voltmeter; an ammeter reading up to at least full load 
intake amperes, and a source of supply SS at the normal voltage 
and frequency for which the motor is built and capable of 
reduction to about full normal voltage at full normal 
frequency, as before, together with— 

For motors with mesh-connected stator —2 similar wattmeters 
having a capacity of about of the full-load output of the 
motor. 



Fig. 101. 


For motors with well-balanced star-connected stator —1 watt¬ 
meter having a capacity of about of the full-load output of 
the motor. 

The ohmic resistance of a complete stator phase (and of a 
rotor phase for reference later, if needed) will be required, and 
can be obtained from a separate measurement with a high- 
reading ammeter and low-reading voltmeter, by the fall of 
potential method {vide p. 84). 

Observations.—(1) Connect up as in Fig. 101 if the motor 
stator is mesh connected or if the stator is star connected but 
not well balanced, and adjust the pointers of those instruments 
which require it, to zero. The terminals of the stator of the 
motor 31 are 1, 2 and 3, whether it is star or mesh connected. 

(2) Close the switch aS^^, /S'g, and start up 71/, finally short- 
















ELECTRICAL ENGINEERING TESTING 


279 


circuiting the starter. Then with the supply at normal voltage 
and frequency note the speed of J/ on the tachometer and the 
I’eadings of all the instruments, the motor running quite light 
and at “no load.’* 


—If Ilie power factor of the system is low, one of the 
wattmeters will read negatively. Reverse the connections to its 
shunt coil and take the reading which must be considered — 
and subtracted from the reading of the other wattmeter to get 
the total true power. 


(3) Open S\y and when the motor comes to rest, clamp the 
shaft in any convenient way to prevent it rotating, and place 
the starter at short-circuit. Now apply any convenient lower 
voltage, say, J to of the normal value at full normal frequency, 
and again read all the instruments as in Test 2 above and switch 
off and open the starter. 

Note. —A lower voltage has to be used in this test for the 
larger-sized motors, because the normal voltage would cause 
dangerously large currents to flow which would probably damage 
the stator winding in even their brief application. The true 
static current will now be the observed current x by the ratio 
of the two voltages, while the corresponding static watts will be 
those observed x by the square of this ratio (§ee table). The 
reason for this is that the static watts are nearly all copper loss 
and hence cc to (current)^. 

(4) Measure in a convenient manner (pp. 84 and 263) the 
resistance between any two terminals of the stator and rotor, 
preferably while warm. Then the resistance of each complete 


stator phase (star connections) 



and for (mesh connec¬ 


tions) r, = Also in the case of a slip ring rotor obtain the 

ratio of transformation given by the ratio of any stator voltage 
to the corresponding rotor voltage with rings open-circuited and 
rotor in the position giving max. volts. 

Record your results as follows— 


280 


ELECTRICAL ENGINEERING TESTING 


Motor: No. . . . Maker . . . 

Rated Full Load: B.II.P. = . . . Line Volts (F) =* • • • Line Amps. = . . . Speed = . . . 
Slip at ,, ,, = . . . Normal frequency = . . , 

Stator Phases connected in . . . 

Resistance:—complete Stator Phase, rs = . . . Rotor Phase, rr = • . . 


Motor running quite freely at 0 Load and Normal Line Voltage and frequency. 

Speed 

of 

Rotor 

by 

Tachr. 

Volts 

across 

Stator 

Phase 

Vo. 

Amperes in 

Wattmeter 

Readings. 

Total 
Intake 
True 
Watts 
Kw 
= ico. 

Power Factor 

Angle 

of 

Lag 

02. 

Line 

Al. 

Each 

Stator 

Phase 

Ao. 

From 

W\ 

U'2 
and 
curve 
(see 
p. 510). 

Cos. 02 
_ ^3 Wo 
ZAlVo' 

Wi. 

^2. 

Wi - W2 

= w. 







\ 





I 


Stator of Motor supplied with lower voltage than normal, but at normal 

Rotor,, ,, at Standstill and Shorted 

Ap¬ 

plied 

Line 

Volts 

Vs. 

Amps, in Line 

Stator 

Amps. 

at 

Volts 

V 

= As. 

Wattmeter 

Readings. 

Total Intake 
True Watts at 

PoAver 

Factor 

Cos. 

_\/3zos 

3VAsi 

Angle 

of 

Lag 

^1- 

At 

Volts 

Vs 

A^l. 

At 

Volts V 

(f.-O 

= Am. 

Wi. 

102. 

loi - W2 

= w. 

Volts Vs 
Kw 
= w^s 

Normal 

Volts 

= Ws. 













Note. —With mesh-connected stators: Volts per phase = 
line volts and amps, per phase = line amps.; with star- 

connected stators: Volts per phase = —p line volts and amps, 
per phase = line amps. 

It will be seen that the Power Factor for the “ no load ” and 
for the “ short-circuit ” tests is found from the relation—■ 


Cos $ = True Watts absorbed per phase 

Amps, per phase x volts per phase’ 

but it is perhaps more convenient to calculate from the experi¬ 
mental readings by means of the fraction given in the above 
table. 

Fiom the data contained in the above tables, the Hey land 
Diagram can be constructed, giving the performance of the 
motor. 































































ELECTFdCAL ENGINEERING TESTING 


281 


Construction of Heyland Diagram. —This will be understood 
more easily by working it out from tests recently made by the 
author on a 360 B.H.P. 500 volt three-phase induction motor, 
running at a speed of 300 revs, per min. with a normal 
frequency = 50 per sec. The stator windings were star con¬ 
nected, the rotor windings being also star connected, and led out 
to three slip rings which were connected to a starting resistance. 
The method of procedure with this motor was as follows— 

With an ammeter in one line and a wattmeter connected 
between the neutral point and one terminal of the motor so as 
to measure the true watts absorbed by one phase (see Fig. 151, 
p. 396), the following measurements were made—• 

Motor running light at normal speed, frequency, and voltage 
with rotor short circuited. —Wattmeter reading = 3600 watts 

per phase = 

Line current (Aj^) =142 amperes. 

Kesistance of each phase of stator (cold) = 0 0122 ohm., or 
by calculation about 0*013 ohm. (hot) on the assumption of a 
maximum temperature rise of 70° F. above that of the air. 
Lesistance of each phase of rotor (cold) r^. — 0'0054 ohm. 

Ratio of transformation in rotor 500 :325. 

Bearing in mind always that the diagram is constructed with 
reference to one phase of the motor, and not the motor as a 
whole. Further that in testing motors with mesh-connected 
stators it would be the total power absorbed that would be 
measured by the two line wattmeter method (Fig. 101^ p. 278) 
instead of that per phase. 

Hence the power factor of any phase, whether in star- or mesh- 
connected stators, can be calculated best from the general relation 



cos. 

4d 




where = total power in watts absorbed by motor running 
light with a line current Aj^ and line voltage F. The numeral 3 
reduces to watts per phase, and reduces the line voltage V 
or line amperes Aj^ to the corresponding quantities per phase in 
the case of star- or mesh-connected stators respectively. Thus 
in the present case 



282 


ELECTRICAL ENGINEERING TESTING 


cos. ^2 




^3.10800 
3.142.500 


0-0878 


or $2 = 85*. 

Motor at standstill with rotor short circuited and stator 
supplied at normal frequency. —^With these conditions we require 
the line current that would flow, and also the true watts absorbed, 
at normal line voltage. As, however, a line voltage as great as 
the normal value would, in most motors, produce an abnormal 
current that would be inconvenient to measure and liable to 
damage the windings, a smaller voltage suflicient to give a 
convenient line current is applied. Thus in the present case 
the line current A^ ~ 404 amps. 

„ pressure Vs = 127 volts. 


Wattmeter reading 


= 8000 watts per phase. 


From which we calculate the following static standstill 
values— 

Line current 

. normal volts 500 

Am =-TT-i— 7 — X 404 = X 404 = 1591 amps. 

applied volts 12/ 

Total watts absorbed 


Ws 


X (3 X 8000) = 372,000 watts, 


whence cos 


/5 _ _ \/3 • 372000 n‘97Ai 

• “ 3A7V " 371591.500 ” ^ 


or = 74-3°. 

Current Circle. —Referring to Fig. 102, draw two lines OE 
giving the phase of the supply volts and OA perpendicular to 
one another, and from the point 0, as origin, set off a straight 
line OG (= current which would be taken by the stator if 
directly across normal voltage), making an angle of 74*3° 
with OE and another line OD (= no-load current) making an 
angle ^2 of 85° with OE. Now choose a convenient linear scale 
of current, e.g. in the present case,’ 100 amps. = 1 cm., and thus 
make 


OG = 1591 amps. = = 15 91 


cms. 


long, 


100 









< 


Fig. 102. 















284 


ELECTRICAL ENGINEERING TESTING 


1 49 

and make OD = 142 „ = = 1*42 cms. long. 

Then with a centre L (on OA) draw a semi-circle ACZD 
through the points G and 2>, and cutting OA in A and F, thus 
determining the point A. From D draw DF~ energy com¬ 
ponent of no-load current perpendicular to OA and cutting it in 
F, then FFO is the no-load triangle of currents OF = the no- 
load magnetizing current cos 0^ = the no-load power factor. 
Rotor current = E'R X ratio of transformation stator intake 
oc and AZF is the current circle for the motor. 

Output Circle.—Join CA, and from A draw AH perpendicular 
to AC, cutting the perpendicular HZ, to OA through L, in the 
point II. With centre H draw the output semi-circle AXF 
through the points A and F. Since the angle CAH = 90°, the 
line CA is a tangent to the output circle, the ordinate of 
which at A is zero, thus satisfying the condition of no output 
corresponding to this point and the point C, output of motor == 
ST which has a max. value = XJ. 

Torque circle.—The torque of an induction motor is pro¬ 
portional to the rotor flux [Rj.) x the rotor current (R^) and in 
fig. 102 the right-angled triangle AGO is a triangle of fluxes in 
which AG oz Rp] AO cc stator flux {Sp) which is also oc applied 
voltage per phase and OG cc leakage flux Lp. Consequently the 
actual rotor flux will he oc AG — copper drop in stator resistance, 
and we proceed by first finding the voltage represented hy AG 
when the volts per phase of stator are represented by AO. 
Thus— 


volts per stator phase _ length of AO _ 16*7 cms. 
volts represented by AG length of AG 4'55 cms.’ 


whence—volts represented by 

AG 4'55/500\ 

AG = -jq X volts per phase = = 78'66 volts. 

Now the copper drop for stator phase (for the short circui 
current As = 1591 amps.) 


= Asr^ = 1591 X 0'013 = 20-68 volts. 

Therefore from the point G mark off along GA the length 
GN =■ 20'68 volts, which is proportional to 









ELECTRICAL ENGINEERING TESTING 


285 




78-66 


20-68 X 4-55 
78-66 


= 1-196 cms. 


Now find a centre II in Eli, such that a semi-circle ANYRF 
described from it passes through the points ANF. This is the 
required torque circle. Since the rotor flux is represented by 
AN, the total torque is oc AN x OF which is oc to the area of 
the right-angled ^iV^7^(the line iVi^not being shown in Fig. 102). 
But the base AF is constant. Hence the total torque is cc to 
the altitude of the triangle. For example the total torque is 
cc TtlF for the stator current OB. Again, in the no load triangle 
of currents OFF, the no load wattless magnetising current = OF 
lagging 90° in phase behind the E.M.F. OF, and having a load 
current component in phase with OE = FD. The resultant 
no load stator current as read off on the ammeter = OB. Hence, 
since useful torque = total torque - torque spent in overcoming 
internal frictions for the same stator current OB, we have 
useful torque oc RW ■— UW oc RU. Starting torque would be 
NM at the starting current OC and a power factor cos 6i. 

Slip of the Motor. —Since we know that the slip is directly 
oc to the rotor current NF, and inversely oc to the rotor flux 
AN, we see that it will have its maximum value at C and 

minimum value at D, for (which is cc to slip) has its 

NA ^ 

maximum value unity at G when the motor is at standstill. 
From G therefore draw CP perpendicular to AK cutting AO 

N F 0 P 

in P. Then the slip oc cc cc QP, since AP is constant 

NA AP 


and the triangles ARF and APQ are similar. 

Now maximum slip corresponding to the point C and the 
motor at standstill is cc CP, which scales 4-35 cms. and 
represents 100 % slip. 

The slip at the output load ST when taking a stator current 

OB is X 100 % = ^ X 100 = 0-9 %. 

length PC 4 oo 

Stator copper loss = [BV — RW) watts and rotor copper 
lo^B =(RU—ST) watts each to a scale of watts suitable for 
input BV and output ST, BR and RS = ohm drop of volts in 
stator and rotor respectively to same scale as OA gives stator 
volts per phase. 








286 


ELECTRICAL ENGINEERING TESTING 


Application of Diagram. —The diagram can now be employed 
for determining the performance of the motor at any load 
corresponding to any point such as B taken anywhere on the 
current curve AZF. Suppose that we choose the point B as 
being the point of contact of the tangent OB with the current 
circle AZF. Join AB, cutting CP in Q, the circle ANF in S, 
and AYF in R, and draw the perpendiculars ST, RUW, and BIV. 

Power Factor. —For any given point on AZF the power factor 
is given by the cosine of the angle between the join of this point 
with 0 and the line OE. At B it has the maximum value 
possible, since OB is a tangent to the circle, namely 

cos. 0 = cos. 32'8° = ’8406. 

Stator Current per phase = OB, which is also the line current 
in the present case since the stator is star connected. 

This scales 4*95 cms. corresponding to 495 amperes (since 
100 amps. = 1 cm.). 

Total Apparent Watts absorbed = . V, line amps. = 

.500.495 = 428,600 watts. 

Total True Watts absorbed = . F. (component of OB in 

phase with and parallel to OE). 

= J'd. V. BY with star con¬ 
nections = ^3.500.416 = 
360,200 watts. 

The same result is given by 3(A V) x volts per phase. 

Stator copper loss for this load = 3 (r^ x component FB‘^ of 
stator current) = 3.0*013. (435)^ = 7381 watts. 

This loss is proportional to BR which = 0*43 cm., and the 
Rotor copper loss is oc to RS which =1*0 c.m., and this loss 

therefore = .^ x 7381 = 17,165 watts. 

4d 

The total loss in the motor at the load corresponding to the 
intake stator current OB = 10,800 + 7381 + 17,165 = 35,346 
watts. 

The output of the motor therefore = 360,200 — 35,346 = 
324,854 watts = 436 B.H.P. 

09i Qrjj. 

The efficiency of the motor therefore = - > = 90*18 7 

360,200 

when giving 436 B.H.P. or 21 % overload with a power factor 
already found of 84 %. 



ELECTRICAL ENGINEERING TESTING 


287 


The ordinate ST thus represents 436 B.H.P. or the scale of 

436 436 


the ordinates of the output circle is_= 

ST 3‘65cms 


= 119*5 


B.H.P. per 1 cm., consequently the rated full load of the motor, 
namely 360 B.H.P., will be given by an ordinate such as S2\ but 
only 3*014 cms. long. 

The efficiency, power factor, and slip, etc., can now be worked 
out for this new full load point which gives another point, such 
as E on AZF nearer to F and a line such as AB, at a smaller 
angle to OA. Hence the performance at any load can be 
determined. 

Further, AT— the torque in pound feet, 
and o) = the angular velocity of the rotor = 27 ^ 9 ^, 

where n = the revs, per min. of the rotor at a given B.H.P., 


then 


B.H.P. = 


<0 


T 


^ttiiT 


or T = 


33,000 B.H.P. 


33,000 33,000 ' 27rn 

Now since at B the B.H.P. = 436 and the slip = 6*9 n = 


300 X 93*1 


= 279 r.p.m. .*. T = 


33,000 X 436 


= 8210 lb. ft. 


100 27r.279 

Hence the scale of the ordinates, such as RUf of the torque 

circle is known from all other ordinates from the above, and 

8210 8210 , 

: 210*5 lb. ft. per 1 cm. 


RU 


3*9 


The maximum torque which the motor can exert before pulling 
up is represented by JY^ and the maximum B.H.P. by JX. 

The starting torque corresponding to the current CO is XAI. 

The Hey land diagram becomes more accurate the smaller the 
no load losses as compared with the copper losses, i. e. the larger 
the motor tested. The performance of the motor is slightly 
better than given by the diagram, while for motors smaller than 
for 4 or 5 H.P. the line AID should be drawn downwards from 
D at an arbitrary angle of about 25° to OA for greater 
accuracy,^ in correcting the small error (slightly afifecting the 
accuracy of the diagram) due to the greater proportion of no-load 
to load losses in small motors. 


^ For higher accuracy see Theory of Induction Motors by Diagram, G. 
Ossanna; Zeitschr. Elektrotech. Wein, 17, pp. 223—248 (1899), and Circle 
Diagram, by J. L. la Cour (same journal), 21, pp. 613-645 (Nov. 1903). 









288 


ELECTRICAL ENGINEERING TESTING 


Note.—If OG be drawn perpendicular to RF produced, then 
AB and OG will always be parallel at all loads and perpendicular 
to BG. 

The sides OB, BG, and GO oi the triangle OBG, represent the 
stator current, rotor current, and magnetising current respect¬ 
ively. Now, in any electro-magnetic circuit the total flux = 

the useful flux the leakage flux, while called 

the leakage factor v, which is always greater than unity, and 

leakage flux called the coefficient of magnetic dispersion a 
total nux 

which should be always much less than unity. 

In the Hey land diagram, Fig. 102, the leakage factor 

OA 


V = 


FA 


OF+FA _OF 
FA “ FA 


and the dispersion coefficient 
OF OA - 


O' = 


OA 


FA _ = 1 _ 1 

OA ~ ^ OA “ ^ v‘ 


(102) Complete Test for Efficiency, Slip, 
Power Factor, and Temperature Rise, 
of Three-Phase Induction Motors. 
(Sumpner and Weekes Method.) 

Introduction. —This method,^ due to Dr. W. E. Sumpner and 
R. W. Weekes, is an application of the principle of the ordinary 
Hopkinson test of a pair of d.c. dynamos (p. 228) to the test of a 
pair of three-phase induction motors. The arrangement, which 
entirely avoids the use of a brake, is extremely convenient for 
obtaining the temperature rise due to a long run at any particular 
load, and is shown in Fig. 103. M and G are the two machines 
to be tested, of which 21 is made the motor and G the generator. 
Their stator terminals T are connected to the main supply SB, 
which is at the normal voltage and frequency required by G and 
21, A. belt B drives the rotor pulleys, which must be of different 
diameters in order to enable the generator G to run at a higher 
speed, and the motor 21 at a lower speed than that of synchronism. 

^ Communicated by the authors to “Section G” of the British Association, 
August 22, 1904, at Cambridge. 







ELECTRICAL ENGINEERING TESTING 


289 


If and Eg are the diameters of these pulleys : then assuming 
the rotors are to remain short circuited during the test, the ratio 

Ejtf 

jj- must be such as to cause the right slip for the load 

required. For example,—if the slip of each machine 31 and G 
= 2 5 % at full load, and if the probable efficiency of each =: 85 %, 
then 15 % of the load is lost in each, and the overload of the 
motor il/ = 30 % roughly, corresponding with 2-5x 2*5 or 
3*25 % roughly. The other machine G working as a generator 
developes full load and has a negative slip of 2*5 %. E.n and Eg 
must therefore differ by 2*5 + 3-25 or 5*75 %, assuming no 
mechanical slip of the belt R on the pulleys. If, however, 1*25 % 
be allowed for this, the pulleys must differ by 5*75 +1*25 or 7 % 
in diameter, and the machines under test will, when switched 
into circuit, take a perfectly definite load which can be maintained 
for any length of time. 

An interesting feature of the test lies in the fact that G, the 
machine used as the generator, gives current of about the same 
power factor as that of the current supplied to the motor df, while 
the current from the supply mains ES is the difference of the 
power components of the machine currents, together with the 
sum of the inductive components of these currents. Consequently, 
the power factor of the main current from SS is very small, and 
may be only |rd of that of the machine current, the main current 
from SS may be equal to, or greater than, that through the 
machines, while the actual power taken from SS may be less 
than of that circulating round the machines G and 31. 

Alteration of load with two given pulleys can be obtained by 
alteration of resistance in the rotor circuits between starter and 
slip rings, or by using a low resistance starter which can stand 
the full load rotor currents. The C^R losses in these rotor resist¬ 
ances, if appreciable, must be deducted from the power taken 
from SS in order to get the total loss in the two machines G and 31 
and in the belt drive. The machines can be run at various loads, 
•\vith resistance variation such as abov’e, if the pulleys are chosen 
v/ith sufficient difference to obtain the maximum slip required. 

The magnitude of the mechanical slip at the pulleys is deter¬ 
mined by the ratio of their circumferential speeds, a quantity 
difficult of determination with any accuracy in ordinary belt 

drives, but most easily found in the present method. Thus— 

u 


290 


ELECTRICAL ENGINEERING TESTING 


let Sj, = frequency of the supply current, 

Gj.’ ~ „ „ generator rotor current, 

Mj,- = 5 , „ motor rotor current; 

then ratio of the rotor speeds R = {Sj'+ Gj.) ~ — and 

ratio of the circumferential speeds of the pulleys = R 

Rjr 

G and can be very accurately determined, and are each small 
compared with so that although Sp cannot be so accurately 
found, the value of R is not much affected by small errors in Sp. 

The belt losses are easily determined, as shown in the table, 
and are caused by (a) extra bearing friction and in bending and 
driving the belt, (5) heating of the pulleys due to belt slip. 

Apparatus.— The two similar three-phase induction motors to 
be tested : suitable pulleys and belt; four alternating current 
ammeters alternating current voltmeter F; four 

wattmeters w^, ^^ 2 * 1 source of three-phase supply SS of 

normal voltage and frequency for which the machines under test 
have been built; three-throw switch s^. 

N.B.— Switches must be used with the stator connections if the 
rotors are of the “ short-circuited ” type, but are not wanted if 
the rotors are supplied with slip rings and starting resistance. 
Only two wattmeters will be needed if one is connected between 
neutral point and a terminal in the case of each machine, since 
in this case total power = 3 x power of one coil, which is sufficiently 
accurate for commercial work. Two wattmeters to each machine, 
as shown in Fig. 103, is, however, the best and most accurate 
arrangement, and has the additional advantage that the ratio of 
the readings of a pair of wattmeters gives the power factor inde¬ 
pendent of the usual method of getting the power factor from 
true watts -P apparent watts. With two wattmeters the reading of 
one will be owing to the low power factor of the supply current. 

Observations. —(1) Connect up as shown in Fig. 103, and 
adjust those instruments to zero which require it. 

(2) With the supply SS at the normal voltage and frequency 
needed for M and G, and the belt R off, close Sg, 53 , and start up 
M and G, which must run in the same direction. If they do not, 
stop them, change the connections at T, and start up again. Note 
the readings of all the instruments, and denote those of and 
by vJqi and respectively. 


ELECTRICAL ENGINEERING TESTING 


201 


(3) Stop M and place the belt on, and start up again, with 
only one of the machines in circuit, and acting as a motor, hut 
driving the other by belt with its stator excited and rotor open 
circuited. Note the readings of all the instruments, and denote 
those of 10^ by Wj^-^ and lOj^^ respectively. 

Thus the belt loss = {lOjn + ~ (^oi + '^ 02 )* 

(4) In Tests 2 and 3 above, and in all future load tests, the 
slip of each machine can most conveniently and accurately be 
determined by measuring the frequency of the rotor currents by 
suitably shunting any ordinary low reading d.c. voltmeter to the 



slip rings of the rotor, and noting the number of periods made 
by the pointer in, say, 20 seconds, these being slow enough to be 
easily counted. If Mp= number of periods per second, in, say, the 
case of the motor rotor currents and Sp = frequency of the supply 
current in periods per second, then the slip of the motor = 

4 ^ 100 %. 

(5) Take readings at different loads, obtained by altering 
the resistance in the rotor circuit and tabulate as shown on 
page 293. 

The values of the losses given in columns 31 and 32 are 
accurate enough for commercial purposes if the two machines 
are of the same make and of about the same rated full-load 
output and if subjected to the same voltage. A greater degree 
of accuracy may be obtained, however, by subdividing these 


































292 


ELECTRICAL ENGINEERING TESTING 


losses. In addition to the diameters of the pulleys indicating 
which machine is the motor and which the generator, the latter 
is the one which must always run at the higher speed. If 
the belt is removed and the machines run light, the wattmeters 
TFj and IFg will read negatively if G is the generator. 

Column 44 is obtained from the curve Fig. 104, which is drawn 
in the following manner. 

The ratio of the two readings of wattmeters Wi and TFg 
connected as shown in Fig. 103, varies from + 1 to - 1. Hence 
the power factor cos. ^ can be calculated from the readings of 
and by substituting different values of cf> (the angle of 
lag) between current and voltage in the formula shown. 




(t 

“o 
go. 



- V; 

lEC 

1 cos 

(30 

+ 









w, 






yk- 

= Vi 

lEC 

!co« 

(30 










(?. 






w, 

CO 

3(3t 

y!). 


/8c 

>S 

- Sll 

i(p 



N \ 



w 







CO 

e(3C 



/3c 


+ sir 

1 (}) 






g 





























































































































































































































































RATIO OF READINGS 
Fiq. 104. 




The value of so found plotted against the value of cos. ^ 
gives the curve, Fig. 104. As an example of the application of 

this curve, let 6800 watts, = 18000, then ^ = 0- 


37 


and cos. <j!, = 077. Again, let ]]\ = {-) 2000 watts, = 


IF 


4000, then O’oO, and cos. = 0T8. 


























































Machine No. . , . Maker . . . Full Load,—Volts . . . Amps. . . . Speed . . . used as Generator Q. 

Machine No. . . . Maker . . , Full Load,—Volts , . . Amps. . . . Si»eed . , . used as Motor M. 

No. . , . Resistance of each: Stator Coil . . . Rotor Coil . . . Diam. of Motor Pulley D at = . . 

No. . . . Resistance of each: Stator Coil . . . Rotor Coil . . . Diam. of Genr. Pulley Da = . • 

In Test No. 2 wqi = • . • W02 = • • . In Test No. 3 Wm = . . . wb2 = • . • 


ELEGTRIGAL ENGINEERING TESTING 


293 



Cl 

S = ^ II 


CO 

CO 


CO 

4) 

m 

in 

O 


o 

H 


u 

o 

"g s 

<v 

O 


Co 

+ Ci3 

II 


(M 

CO 


I CO 

^ iCb • 

^ II 


IN 


u cq 
o ^'d I i 

5 Cl 

- s 
■g a>,+ 


PM 


o 

CO 


4) 

4^ 

Cw 

o 

d 

U 

o 


00 

4) 

O 

to 


c3 

U 

4> 

& 

s 

4) 

H 


Generator 

Stator. 

29 

^ c 

00 

SB 


30 


Air. 

i 

27 


W 5? ® 

S ^ 

M , Ttx 

40 * r-«^ 

»—I O II 

^ C; g II 

pq g Y 

^ I 


o 

(M 


O 

(/} 

o 

3 

4> 

ci 

to 

cS 


o 

(4 

4-3 

O 

a> 

s 


o , 

^ VP 
^-1 rH OV 

fl V ^ 

4> . <>5 

a H 


o 

o 


M Cs. I t," 

II 


\o 


(M 


^43 rfc I 5S 

d;Scih 


CIS <« fc. 

•g ® PS 
CO 


X 


^5q 


It, 

I 

fc. 

V) 


CO 

Cl 

















































































































294 


ELECTRICAL ENGINEERING TESTING 


(103) Relation between Efficiency, Slip, 
Torque, Load, etc., in an Induction 
Motor with Variable Rotor Circuit 
Resistance. 


Introduction. —The present test is obviously just an extension 
of, and similar in almost every way to, test No. 100, which 
is therefore repeated here but with different amounts of the 
starting resistance (r) in circuit instead of it being all cut out 
to short-circuit as in that test. Consequently there will be 
one set of curves, such as was obtained in test No. 100, for each 
different value of rotor circuit resistance used in the present 
investigation. 

Further, if, say, five different rotor circuit resistances were 
used, giving five complete sets of curves as in test No. 100, then 
any of the variables plotted, say, against load, for the different 
constant rotor resistances can be transferred and replotted against 
rotor resistance, e. g. there would be five efficiency-load curves, 
then if a straight line was drawn through, say, full-load point, 
parallel to the axis of efficiency, and cutting the five efficiency 
curves; the five different efficiencies obtained by the five inter¬ 
section points can be plotted against the five values of rotor 
resistance of the efficiency-load curves to give a curve of five 
points between efficiency and rotor resistance only. 

If Eg = E.M.F. induced in each stator circuit due to the 
rotating field, 

= number of turns per phase in each stator and 
rotor winding respectively, 

= angular velocities of rotating stator field and rotor 
respectively, 

Tji = resistance of each rotor circuit, 

Lji = self-induction of each rotor circuit, 

2) = number of pairs of poles in the rotating stator field, 
f = frequency of the stator supply voltage and 
currents, 

Ji = the slip. 


Then we have K = -I- - and the frequency of the induced 




E.M.F. and currents in the rotor circuits will = 


®1 




ELECTRICAL ENGINEERING TESTING 


295 


Kf ~ slip cycles per sec., and it can be shown that the 

running torque T is given by the relation 

i\Vaq(/iX27r/Z^)2+r/) AV2^\K\27rfL^)^ + r/) 

p 

where fjp = speed of the stator rotating field in 

revs, per sec., 

and = wj = its angular velocity, 

E27rfLji = reactance per rotor circuit when in 
motion and which is oc to slip, 

and K^{2TrfLji)- = (impedance)^ per rotor circuit when in 

motion. 

Further, if = current per phase in the rotor, and F — 
leakage flux, then the coefficient of self-induction per phase of 

the rotor = \ ^ henries, which can also be calculated from 

lU Aji 

the shape of slots and winding in them. 

The above expression for T shows us that the running torque 
T of the induction motor is oc to the square of the stator 
voltage, i. e. to the square of the stator flux, and increases 
as both the supply frequency and reactance per phase of the 
rotor decreases, becoming a maximum {bd) when 


K = 


__ O ^ ____ _ 

27V ma..~ Ns^'27£2{27vfLn) 

P 



Fig. 105. 


Thus, since the last expression for the maximum value of the 
running torque does not contain rji^ we see that it is constant 
and independent of the rotor circuit resistance, but for different 
values of rotor resistance will attain tlie same maximum value 
hd at a different slip, as indicated in Fig. 105. The motor starts 
















296 


ELECTRICAL ENGINEERING TESTING 


at 0 with 100% slip and reaches the same maximum running 
torque bd ~ og for slips of C(i% and co = 100% {i. e. full speed) 
respectively, with rotor circuit resistances and ; since the 
stator current depends on the constant no-load current and 
rotor current, and the latter will always be the same for a 
given torque, it follows that .each value of torque will have 
a definite stator current which is independent of the rotor 
resistance. 

Apparatus.—Precisely that for the preceding efficiency-load 
test No. 100, the starting resistance in the rotor circuit being 
of sufficient current-carrying capacity to enable it to carry, 
without overheating, the full-load rotor currents. 

Observations. —Connect up as in Fig. 100, and carry out 
the tests precisely as directed in that test, for as many different 
values of starter resistance as possible. 

Tabulate as shown on p. 276, adding two extra columns for 
values of starter resistance (r) and total rotor circuit resistance 
Tji = [r^jo + r) respectively. 

Plot the following curves having torque T in lb.-ft., and rotor 
current as ordinates with percentage of full speed (or slip), 
and stator current Ag respectively as abcissss for each value of 
rotor circuit resistance (r^). 

Also curves having efficiency, power factor, slip and true 
stator watts absorbed as ordinates with B.H.P. as abcisste for 
two widely different values of 

Inferences. —From a careful study of the numerical results 
and curves state clearly what can be deduced. 


(104) Relation between the Starting Torque, 
Current, Voltage, and the Rotor Circuit 
Resistance of an Induction Motor, 
(Rotor at Standstill.) 

Introduction. —Under these conditions the slip will be 100%, 
since the speed is zero. The power absorbed will include both 
iron and copper losses, and the motor will approximate to a 
static transformer with a non-inductive secondary load. 

Now the frequency / of the stator supply will also be that 


ELECTRICAL ENGINEERING TESTING 


297 


of tho rotor circuits when at rest, and if we put ^ = 1 in the 
expression for the running torque (p. 295), since the rotor is 
now stationary, it will be seen that the starting torque Tq is 
given by the relation 

■0 AV27r/(2^/X4^ + 

P 

where ^ = speed of the rotating field in revs, per sec. and AiHl 

p 

its angular velocity, square of the im¬ 

pedance, and ^Tr/Lji = the reactance per phase of the rotor 
when at rest. From the above relation we see that the starting 
torque 2'q is x to the square of the stator voltage, i. e. to the 
square of the stator flux, and increases as both the supply 
frequency and reactance per phase of the rotor decreases, 
becoming a maximum when 27rfLji = 

For this last condition— 




iJj 


we therefore have the following most important deductions, 
namely, that for a given supply frequency, the starting torque 
is a maximum when the resistance and reactance of the rotor 
circuits are equal and each as small as possible. Since the 
rotor currents are a maximum at starting, the present test 
enables the maximum value of the starting resistance to be 
obtained under either of two conditions: namely, (1) for 
maximum starting torque, or (2) for maximum safe starting 
rotor current. In the former, by measuring the values of 
and 7 '^ per phase winding of the rotor we know that for maximum 
starting torque 27r/Z^ must = + r, whence the external starter 

resistance must have a maximum value r = — Vip ohms 

per phase. 

In the latter, if = the standstill slip-ring voltage at 

normal stator volts and frequency, then = the standstill 

V3 

volts per phase winding, whence r = . ohms per phase for 

V '^Aji 

a maximum safe starting rotor current A^. The gradation of r 










298 


ELECTRICAL ENGINEERING TESTING 


between this maximum value and 0 depends on the number of 
switch contacts and sections chosen. 

Apparatus. —That detailed for the no-load short-circuit test 
No. 98, using an induction motor having a slip-ring rotor 
connected to the usual form of three-phase equal variable 
starting resistance of a current-carrying capacity sufficient to 
allow the necessary time for taking readings without over¬ 
heating. In addition, a block brake and lever, preferably similar 
to that shown in Fig. 95, will be needed to measure the torque 
exerted by the shaft. 

Observations. — (1) Connect up as in Fig. 100, levelling and 
adjusting to zero such instruments as need it. On starting up 
see that all lubricating arrangements are feeding properly. 

Starting Torgite with Rotor Circuit Resistance for a Constant 
Supply Voltage and Frequency. —(2) Adjust the supply frequency 
f to the normal value for the motor and the supply voltage Vg 
to some convenient value, if necessary lower than the normal 
value for the motor in order to avoid excessive rotor currents 
and keep both constant. Then read the spring balance and all 
other instruments as quickly as j^ossihle^ when (r) is moved one 
contact stud at a time from its “full in” position to such a 
position nearer that of short circuit at which the rotor current 
reaches a safe overload value. Finally measuring the resist¬ 
ance of each rotor circuit corresponding to each contact-stud 
position. 

Starting Torque with Supply Frequency for Constant Rotor 
Circuit Resistance and Supply Voltage. —(3) With the supply volt¬ 
age Vg and starter resistance r adjusted to convenient values for 
giving safe maximum rotor currents and kept constant, read 
the spring balance and all the other instruments as rapidly as 
possible at each of a series of supply frequencies (/) between 
the maximum and minimum values possible and convenient. 

Starting Torque with Supply Voltage for Constant Rotor Circuit 
Resistance and Frequency. — (4) With supply frequency and starter 
resistance r adjusted to convenient constant values, read the 
spring balance and all instruments as rapidly as possible at 
each of a series of supply voltages Vg between maximum and 
minimum values giving safe maximum rotor currents, and 
tabulate all your results as follows— 


ELECTRICAL ENGINEERING TESTING 


299 


Motor: No. . . . Type . . . Maker . . . 

Full load : Volts = . . . Amps. = . . . Speed »* . . . Frequency = . . . 

Res. per phase winding of rotor r„, = ohms. 

Leverage of Spring Balance from Shaft Centre (i) = ft. 


Balance Pull 
ir lbs. 

j Starting Torque, 

I To=JF.l lb.-ft. 

Frequency /. 

Volts Fg 

CO 

a 

S 

<1 

True Watts. 

OQ 

S'? 

>- 1m 

a.> 

ft 

< 

S ^ 

a 11 ^ 

0 ^ 

52 

Angle of Lag <j)- 

Rotor. 

Values of Vg'^^ 

frit 


OJ 

-fl 

05 

•3 41 

0 

^ 11 

m 

-4-3 

0 

Kl 

cn 

A 

Scarier 

Rea. r. 

T.-tal Cir¬ 

cuit Res. 

= ^to + r. 









1 








(5) Plot curves having values of As, A^, and Tq as ordinates, 
with values of (r^) from obs. 2 as abscissse, also curves having Tq 
as ordinates with («) frequency (/) from obs. 3, and (b) statjr 
supply volts (F^) from obs. 4, (c) stator amps. Ag, and (d) rotor 

amps. A An additional column for values of 7q -f- () might 

V/r^/ 

be added to this table in order to see how nearly this ratio is 

V 2 
' s 

fi'R 

Inferences. —From a careful study of the numerical results 
and curves, carefully point out all that can be deduced. 


constant, i. e. how nearly the starting torque is x to 


(105) Determination of the Efficiency, B.H.P.., 
and other Characteristics of Single 
Phase Alternating Current Commutator 
Motors. 

Introduction. —The comparatively small starting torque of 
the induction motor to that necessary for electric traction work 
has led in recent years to the production, improvement, and 
utilization on an increasing scale of the so-called alternating- 
current commutator motor. It is well known that any ordinary 
direct-current series or shunt-wound electric motor will run in 
one and the same direction whichever way the supply current 
flows through it, for with every reversal of the supply current, 
the magnetization of both field and armature will also be simul¬ 
taneously reversed, and the motor will continue to run as if 
nothing had been changed. From this it follows that any 
ordinary D.O. machine will run as a motor when supplied with 




































300 


ELECTRICAL ENGINEERING TESTING 


A.C., though inefficiently owing (1) to the large eddy-current loss 
due to heavy eddy currents which would be set up in the solid 
field system by reason of the rapid reversal of the magnetization, 
and (2) the demagnetizing effect of such currents on the field. 
If, however, the field system is well laminated, like the armature 
of any machine always is, the machine would run with reason¬ 
able efficiency on an A.C. supply, but will develop less power 
than when run with D.C., of the same mean voltage, owing to 
the smaller current and flux, and to the larger internal losses 
due to eddy currents and hysteresis resulting from an A.O. 
supply. 

The field magnets of A.C. commutating motors are either 
bi-polar or multi-polar, whether of the projecting-pole form used 
in D.C. machines, or of the cylindrical form with uniform air-gap 
as used in induction motors, and with definite polarity produced 
by the windings but not otherwise so evident. The armature, 
however, presents the usual appearance of D.C. forms, although, 
along with its commutator, embodying features mentioned later 
and necessary for ensuring satisfactory operation. 

These features will be appreciated after a brief consideration 
of the actions taking place in the machine, but at the outset it 
should be realized that a single-phase series-wound commutator 
motor, built on the best possible lines for a given voltage supply, 
will operate in every way as well, but even more efficiently when 
run from a D.C. supply of the same voltage. In fact, such 
motors have to run on A.C. in some parts and on D.C. in other 
sections of certain tramway undertakings. 

Now, considering such a series-wound motor with (for 
simplicity) a two-pole field, and hence with one pair of brushes, 
with a D.C. supply, producing a unidirectional field in the 
poles and through the armature, there will be set up a uni¬ 
directional induced potential difference (P.D.) having its 
maximum value between the brushes, i. e. along the “diameter 
of commutation” which, with the motor running light, will be 
coincident with the “neutral axis” and perpendicular to the 
direction of the fixed field. This induced P.D. (or “back 
E.^LF.”) is set up solely by reason of the forced rotation of 
the armature conductors across the field, by the supply current 
flowing in them, and with a given field is entirely due and 


ELECTRICAL ENGINEERING TESTING 


301 


directly oc to the speed (n) of rotation. On the other hand, 
with an A.C. supply producing an alternating field in the poles 
and through the armature^ there will be set up two distinct 
alternating P.D.s: namely, (1) the induced P.D. having its 
maximum value between the brushes exactly as mentioned 
above, and with a given field entirely due and directly cc to 
the speed (?i) of rotation; it is in phase with the field and also 
practically with the current, and consequently not in direct 
opposition of phase with the supply E.M.F., and (2) the self- 
induced P.D. having its maximum value between two points 
in the armature winding on a diameter perpendicular to the 
diameter of commutation. This self-induced P.D. is set up 
solely by reason of the transformer action due to the armature 
conductors cutting the alternating field, and will lag in phase 
behind the field flux by an angle of 90°. Its magnitude will 
depend only on the strength and rate of reversal {i. e. the 
frequency/) of the alternating field, and in no way on whether 
the armature rotates or is stationary. It has no effect on the 
action of the motor, nor on the supply ; consequently, due to the 
main field, in the rotating armature of a single-phase commutator 
motor, there are induced two entirely distinct E.M.F.s—one 
caused only by and directly cc to the speed of rotation, the 
other caused only by transformer action and directly oc to the 
supply frequency. 

Now, when a current flows through the armature, the latter 
becomes a powerful electro-magnet, the two halves of the wind¬ 
ing in parallel between the brushes producing two similar semi¬ 
circular electro-magnets having a consequent north and a 
consequent south pole situated in the diameter of commutation, 
and at a distance apart equal to the diameter of the armature core. 
The flux of this armature magnetization will be in phase with 
the current, and have a direction therefore perpendicular to the 
main field flux, or in line with the diameter of commutation, 
giving rise to the phenomenon commonly known as armature 
reaction. It will react on the main flux in three ways : (1) by 
distorting and dragging it round in the direction of rotation, 
(2) by inducing in the armature conductors, as they rotate 
through it, an E.M.F. along an axis parallel to the main field, 
but which will not in any way affect the action of the motor, 


302 


ELECTEICAL ENQINEERINQ TESTING 


(3) by inducing, througli transformer action on the armature 
conductors, an E.M.F. of self-induction 90° in phase behind 
the current and acting along an axis joining the two brushes. 
The value of this self-induced or reactance voltage of the armature 
is La^TrfA where ~ coefficient of self-induction of the arma¬ 
ture winding carrying a current A, and f = the frequency of 
the current A^ which in this case is that of the supply to the 
motor. Since the motor is series wound, the same current A 
will flow in the field-winding which will have a coefficient of self- 
induction Lp. Consequently the series field coils will introduce 
into the circuit a self-induced, or back, or reactance voltage 
= Lp^lirfA. Thus the total reactance voltage of the motor 
will = 2iTfA(^Lp-\- La). Now the reactance of the machine has 
the disadvantage of reducing the power factor of the circuit, 
and should therefore be minimized as far as possible. 

That due to the field coils cannot be reduced, because the 
, chief cause of its existence, viz. the flux, is also necessary for 
the operation of the machine as a motor. 

The reactance of the armature can, and is, compensated for 
by an additional winding on the field system midway between 
the main field windings, and producing a flux equal and opposite 
to the reactance field of the armature, and which is connected 
either in series with the circuit or short-circuited on itself. In 
either case the effect is the neutralization of the armature reaction 
flux and reactance and an increase in power factor. Again, 
although the self-induced voltage in the armature coils, due to 
transformer action and main field has no effect on the action of 
the motor, it has an effect on the commutation. For example, 
an armature coil undergoing commutation is short-circuited by 
the brush while inactive, i. e. generating no E.M.F. by reason of 
its rotation across the field. Since, however, in an A.C. motor, 
the coil by transformer action has, during commutation, a self- 
induced E.M.F., this will produce in it, when short-circuited 
by the brush, a heavy current which when broken as the segments 
leave the brush will cause sparking and the deterioration of the 
commutator. 

Now, the self-induced E.M.F. of a coil decreases with a 
decrease in the number of turns, and if the circuit of the coil is 
broken before the current has time to attain its full value the 


ELECTRICAL ENGINEERING TESTING 


303 


spark will be decreased. Hence in commercial single-phase 
commutator motors, sparking is minimized by [a) having as 
few a number of turns per armature coil as possible, (6) an 
increased number of coils and peripherally narrower commutator 
segments and brushes, (c) as small a supply frequency as 
possible, [d) bruslies of special composition. The narrower seg¬ 
ment in [h) reduces the time during which an armature coil is 
short-circuited and reduces the short-circuit current in it. 

The single-phase A.C. motor possesses much the same 
characteristics as the H.C. form, being a variable speed motor, 
giving maximum torque on starting which decreases with increase 
of speed, and is oc to armature current, but independent of 
power factor. It will tend to race in speed on suddenly re¬ 
moving the load. Further, since the current is simultaneously 



reversed in armature and field, the torque will be undirectional 
though pulsating. 

Apparatus. —An A.C. supply E, preferably a motor-driven 
alternator having a speed and field control independently 
variable between wide limits; ammeter A', variable non-in¬ 
ductive rheostat li, ] frequency meter (/); voltmeter V with 
two two-way keys K-^ K^, 1 wattmeter W; switch S ; and single¬ 
phase commutator motor, to be tested, of which the series field 
windings are F and (a) the armature. 

Observations. —(1) Connect up as in Fig. 106, levelling and 
adjusting to zero such instruments as need it. N.B.—With a 
town’s supply for E, the rheostat R will be needed to start up 
M and for regulating the current afterwards, otherwise with a 
motor alternator this may be done by field excitation. 

(2) With R or the alternator field rheostat full in, and the 
armature shaft clamped to prevent it rotating, close S and adjust 
the frequency f to the normal value for the motor. Now 








304 


ELECTRICAL ENGINEERING TESTING 


gradually raise the voltage until A reads the full-load current 
of the motor, and note the readings of /,' A, W and V (when 
switched by across (a), (F), and (a-\- F), giving readings 

Fa, Vp and V respectively). 

(3) For this same value of V and f unclainp the armature 
shaft so as to see what all the instruments, including tachometer 
Fa, Vp and F, will read when the speed has risen to a constant 
value (which must not be excessive), the shaft running quite 
“light.” 

(4) With normal frequency, and the motor running perfectly 
“light,” read all the instruments and speed at each of a series 
of voltages F between 0, 25% above normal value. 

(5) Repeat obs. 4 with constant normal voltage and wide 
variation in frequency. 

(6) With the normal frequency (/) and the motor running 
light, alter V so as to obtain normal speed on the tachometer, 
and note the readings of /, A^ TF, Fa, Vp and V. 

(7) With this same value of speed and f load up the motor 
to about 25% above full load in some eight or ten successive 
steps, noting the readings of all the instruments, the speed 
being kept constant by raising the voltage F. 

(8) With the motor running light at constant normal 
frequency, obtain the maximum safe speed allowable, note this 
and also the values of F, A and IF; next apply about ten 
different braking loads up to about 50% above normal, noting 
the values of the speed F, A and W at each—F and f being kept 
constant. 

Tabulate all your results as follows— 


Motor Tested: No. ==*.., Type . . . Maker . . . Weight = . 

Full load: B.H.P. = . . . Amps. = . . . Volts =■ . . . Speed = - . . Frequency = . . ! 
Resistance: Armature (worm) r„ = ohms. Series coils (worm) F =» ohms. 


Brake 

Pulls 

• 


9} 

V-c 





r-H 




bo 

OS 

<v 

<v 

Oi 


cS 

V 

d 

s 

GO 

Hi 


x> .O 
P —' 

cr J- 
^ 1 


Voltages 

across 


P 






4> 

3 


Impedance of 


s x: 

■go 


73 .P 

■3 ° 


cc 
V C 


Watt¬ 

meter. 


tc 

.9 

oS 

o 


CO 

'S 

a 


I ^ 


s 

a 

A 

<1 




Total H.P. 


-a fe. 
a; 

i II 

<1 K| 




— 

S I 

« h 


O 

o 


>> 

o 

c 

(V 

*s 

e 


















































ELECTRICAL ENGINEERING TESTING 


305 


(9) Plot the following curves— 

From obs. 4 between voltage V as abscissje with speed A and 
W as ordinates. 

From obs. 5 between frequency f as abscissa) with speed A 
and ir as ordinates. 

From obs. 6 and 7 between loads II^ as abscissre with values 
of % cos cfi, II and A as ordinates. 

From obs. 8 between speed as abscissje with values of 5 
and IIj^ as ordinates. 

From obs. 8 between torque as abscissa) and values of speed 
and A as ordinates. 

Inferences. —State clearly all that can be deduced from the 
tabular results and curves. 

(io6) Relation between the Field Excitation 
and Armature Current, or the ‘‘V’' and 
other Curves, of a Synchronous Alter¬ 
nating-Current Motor Running Light or 
at Constant B.H.P. 

Introduction. —All alternators, whether single or polyphase, 
are reversible machines, and will run as motors synchronously 
v>dth the periodicity of the A.C. supply to their armatures, the 
field system being in all cases supplied with a separate source of 
direct current. 

Synchronous motors are, howe\'er, noi self-starting, for at every 
succeeding rapid reversal of the A.C. supply, the armature coils 
receive equal impulses but in opposite direction, and hence there 
’is no resultant torque. If, however, the motor is first started 
up and run by some other driving source of power, at such a 
speed that any armature conductor passes through the distance 
between the centres of two poles {i. e. the pitch) in half the 
periodic time of the A.C. supply, then on switching it on to the 
supply it will continue to run as an efficient A.C. motor in dead 
synchronism with the supply frequency, irrespective of load, so 
long as this is not sufficient to pull it out of step with the supply 
current. 

A single-phase synchronous motor therefore develops an alter* 

X 


306 


ELEGTIUGAL ENGINEERING TESTING 


nating armature polarity and torque which reverses with the 
rapidity of reversal of the A.C. supply, thus producing unidirec¬ 
tional rotation. On the other hand, the currents in the phase 
windings of a polyphase synchronous motor combine so as to 
form a constant polarity of fixed position relatively to that of 
the field, so causing a unidirectional torque and rotation. 

Apparatus. —Sources of A.C. supply to synchronous motor 
if, and of D.C. supply to starting motor (m) and field of M; 
switch S', lamps A.C. ammeter A, voltmeter wattmeter 

TF; D.C. ammeter a, voltmeter Field ammeter «/, rheostats 
rh and rj, switches Sj and Sm^ with starter or main variable 
resistance (r). 



Note. —The lamps should be stamped for a voltage, each 

equal or even 10% higher than that of if, so as to avoid burning 
them out while synchronising. 

Observations.—(1) Connect up as shown in Fig. 107, levelling 
and adjusting to zero such instruments as require it. On start¬ 
ing the machines, see that their lubricating arrangements are 
working properly. 

(2) The Synchronising or starting up of the A.C. motor under 
test can be effected as follows ; with S and Sj open and (r) off, if a 
starter, or “full in,” if a variable rheostat, close Sm ^-nd operate 
(r) so as to start the machines up to about the normal speed 
of Jf; now close Sf and adjust r, rh and Vj until F indicates the 
same voltage as that of the supply E^, and the lamps L^L., cease 
to blink and go out definitely with a slow period. At this 
moment close S and open S,y^, when the A.C. machine ilf will 
continue to run as a synchronous A.C. motor at a speed entirely 
governed by, and directly proportional to, the supply frequency. 




















ELECTRICAL ENGINEERING TESTING 


307 


Note. —At the above moment of closing S, the back E.M.F. 

( V) of M will be not only practically equal to, but also exactly 
opposite in phase with [i.e. differ by 180° from), that of the 
supply E^. 

The starting up may also be effected by one of the special 
forms of synchroniser now made for the purpose, e. g. the 
rotatory type or synchroscope of Messrs. Everett, Edgcumbe & 
Co., the characteristics of which are as follows : with the supply 
and the motor connected to the respective pairs of terminals 
on the synchroscope, the speed and field of M are adjusted until 
the frequency of M = that of the supply (indicated by the 
rotating 'pointer coming to rest), and the voltage of M is equal and 
opposite in phase to that of the supply (indicated by the pointer 
taking the vertical position ); under these conditions, the dial will 
show a white light and S can be closed. Briefly, therefore, 
close main switch when pointer stops vertically and white light 
shows. If M is running too fast, the pointer rotates clockwise 
and a red light shows, whereas if M is running too slow, the 
pointer rotates counter-clockwise and a green light shows. 

Two- and three-phase machines are synchronised by the same 
single-phase instrument with its 2 pairs of terminals connected 
across any one phase, either side of the main switch contacts 
of that particular phase. With the motor M under test running 
synchronously with the A.C. supply, the following very interesting 
and important investigations can be made, namely— 

(3) With Sm open and r and rh “full in,” M will (unless 
coupled to and released from m by an electro-magnetic clutch) 
simply be turning it against the small windage, brush, and 
bearing frictions, and will therefore practically be running light 
itself. For this no-load condition at normal supply frequency 
and voltage adjust rj to obtain minimum reading on A, and note 
simultaneously that of V, W, aj and the speed. 

(4) Next vary rj, and hence the exciting current (a), by a 
series of steps, above and below the value found in obs. 3, as will 
raise A to a value not exceeding 25% over load, in each case 
noting F, W, «/, A and the speed at each excitation. The supply 
voltage V and frequency being kept constant throughout at the 
value of obs. 3. 

(5) Repeat obs. 3 and 4 for constant B.H.P. load outputs 


308 


ELEGTBIGAL ENGINEEEING TESTING . 


from M of say J, f and full load respectively at the same 
constant supply volts and frequency^ taking that value of a^ 
giving minimum main current A as the starting point of the 
“up and down series” of (cij). 

Note. —The brake load can most conveniently be taken up 
electrically in the coupled D.C. starting motor m by causing it 
to act as a D.C. generator and send current through a suitable 
current rheostat to be connected in series with a switch (neither 
shown in Fig. 107) across the points P and Q. In this case (r) 
must be short-circuited and special precaution taken to keep Sm 
open. 

The product v .a ~ the power absorbed in the added rheostat, 
and, if the efficiency of M is known, the actual B.H.P. developed 
by M is at once obtainable, otherwise with constant excitation - 
of (m), the power absorbed by it will be roughly cc to the 
currents {a) developed, and therefore to the B.H.P. given by Af. 
Tabulate all results as follows— 


Synclironous Motor: No. . . . Milker . , . Type . . . 

Full load (normal) : B.H.P. = . . . with Amps, at . . . Volts, and Speed =» r.p.in. 

D.C. Starting Motor; Full load Amps. = . . . Volts. = , . . Speed = . . . 

Efficiency 2 = at . . . load. 


•Supply. 

■/! 

Pi 

S 

P es 
+3 

‘3 

Armature. 

Power F’actor 
cos (f> = W/A V. 

Angle of Phase 
•Ditt'ereuce 0. 

Speed in r.p.m. 

D.C. Starting 

Mutor as load. 

Frequency/. 

Voltage V. 

Amps. A. 

Apparent 
Walts A V. 

True Watts 

W. 

Volts V. 

Amps. a. 

1 

'5 

.X n 

fH s 
a « 











— 


(6) Plot curves for running light and for each load on A1 
having (1) amps A, (2) power factor (cos </>), and (3) watts W 
as ordinates with exciting current {aj) as abscissae in each case. 

Inferences. —Prom a careful study of the shapes of the curves 
and of the tabular results, state wdiat can be deduced. 


































ELEGTRIGAL ENGINEERING TESTING 


309 


(107) Efficiency and with other Char¬ 

acteristics, of a Synchronous Motor run 
from a Constant Voltage and Frequency 
Supply at Constant Excitation. 


Introduction. —In view of the peculiar relations existing 
between the excitation and other factors as determined in the 
last test, and the use of the synchronous machine for raising 
the average working power factor of a supply system, it is both 
interesting and important to see what effect load has on the 
same factors. This is apparent when determining the efficiency¬ 
load curve of the machine in the present test. 

Apparatus. —Precisely that for test, No. 106. 

Observations. —Carry out obs. 1, 2, and 3 of test. No. 106, 
exactly as stated. 

(4) With the supply voltage Vand freipuency f each kept constant 
at the normal value for the motor i/, and with the excitation [aj) 
kept constant at the value noted in obs. 3 (namely that giving 
minimum armature current A), take a series of brake loads on 31 
rising by about equal amounts up to about 25% over load, 
noting the readings of F, IF, Uj, A speed, and output factors at 
each load. 

Note. —As the heating of a machine is the factor limiting the 
maximum safe output, it is desirable to estimate the brake 
loading by roughly equal amounts of main current A up to about 
25%, or even 50%, over load if kept on only a few minutes, 
calculating and taking the B.H.P. corresponding to such current 
values. The method of loading the motor 31 may be that 
indicated in the Note, obs. 5, test No. 106. 

(5) Repeat obs. 4 for two or three higher—and two or three 
lower— constant values of excitation [a^ than that used in obs. 4 
above, which gave minimum value of ff, and tabulate as per 
schedule shown on page 308, but adding one extra column 

for efficiency (— 

\ E.H.P. absorbed/ 

(6) Plot the following curves, namely, having in every case 
B.H.P. outputs as abscissae with (1) efficiency, (2) amperes ff, 
(3) watts IF, or E.H.P. absorbed, and (4) cos as ordinates 
respectively. 



310 


ELECT RIGAL ENGINEERING TESTING 


Inferences. —From a careful study of the figures and also of 
the shape and relative dispositions of the curves, state what can 
be deduced. 

Relations between the Supply Factors of an 
Alternating Current and the Constants of 
the Circuits supplied. 

General Remarks. —Every electrical circuit possesses three 
distinct qualities, namely— 

(i) Electrical—or ohmic resistance, depending on the length, 
sectional area and material of which it is made. 

(ii) Electrical—or electrostatic capacity, depending on the 
length, surface, form and the specific inductive capacity of the 
surrounding dielectric. 

(iii) Electrical—or self-inductance, depending on the shape, 
form and magnetic permeability of the surrounding conducting 
material. 

All of these qualities are always present in every circuit what¬ 
soever, but it may happen that one or more of them are so small 
as to be negligible from a practical point of view. Thus we are 
accustomed to speak of some special circuit as possessing onl^ 
one of them, any two, or all three of them at once. It is often 
of the utmost importance to know the nature of a circuit, with 
reference to the above qualities, when alternating currents are 
employed, for the presence of one or more of them in such a 
circuit may be very troublesome or may be a necessity according 
to circumstances. Theory dictates that variation in the period¬ 
icity of the alternating supply causes, in some cases,- a consider¬ 
able change in the working results of a circuit, and it is with a 
view to clearly elucidating the effects of variation of frequency on 
circuits in which one or more of these qualities predominate that 
the following tests have been devised, and also of determining 
how such variations affect the power absorbed in the circuit, and 
also the corresponding variation of temperature (if any). In all 
cases the power is to be measured by a Wattmeter as nearly non- 
inductive as it is possible to have it, for it will then give a true 
measure of the power absorbed. The results to be expected, as 
dictated by theoretical considerations, are as follows— 


ELEGTBIGAL ENGINEERING TESTING 


311 


Let A = ^mean sq. value of current in amperes flowing in the 
circuit. 


„ voltage acting on the circuit. 


V = ^mean sq. ,, 

It — ohmic resistance of the circuit. 

L = its self-induction in henries. 

C = its electrostatic capacity in farads or (7 x 10® micro¬ 
farads. 

p = the angular velocity of the alternating supply = 27r x 
frequency. 

Tlien we have for circuit possessing— 

R only :—A — 

and the current is in phase with the voltage. 

V 

R and L only (in series ):—A = y--- 

^ ‘ -f 

the current now lagging in phase behind the voltage by an angle 


6 such that tan. 0 = 


_ 


R 


V 


R and C only (in series) :— A = // 1 V 

"S\GpJ 


+ R^ 


the current now leading in phase in advance of the voltage by an 

1 


angle 6 such that tan. 6 — 


GpR' 

R, L, and C (in series) :—A = 




where L predominates over G, the current now lagging in phase 


behind the voltage by an angle 6 such that tan. 6 


r 1 


Cp. 


R 


where C predominates over L, the current leads in phase in front 
of the voltage by an angle B and the last two relations become— 


A = 


V 


M - ^ 


and tan 0 = -jz - Lp. 

Cp 

















312 


ELECT RIGA L ENGINEERING TESTING 


C (only):— A = — = CpVt 
Cp 

the current now being 90° in phase in advance of the voltage. 

L (only) ■.—A = I, 


the current lagging 90° in phase behind the voltage. 

= Impedic Resistance. 

The radical denominators in the three expressions for A are 
termed the apparent or effective resistances of the circuit con¬ 
taining those particular qualities, though one of these, namely 
JL^p"^ + is very generally termed the impedance of the 

circuit. The terms Lp. -i- and ( Lp -are called the 

Cp^ \ CpJ 

reactances or reactive resistances of the circuit, and when multi¬ 
plied by the current give the reactance voltage. The phase 
relations are shown by the vector diagram OBD^ Fig. 108, and 


D 



if each voltage is -y by the current we shall get a proportionate 
Scalar diagram (^. e. without arrows) in Avhich OD = apparent 
resistance, OB = ohmic resistance, and BB = reactance or in¬ 
ductive resistance, while OA = the current. It will now be 
obvious that if the ohmic resistance is extremely small (for it 
cannot be zero) the impedance becomes = the reactance, but 
when the ohmic resistance is large, the impedance is affected by 
it considerably. Again, since the term for reactance contains 
p = 2 7T X frequency, it is directly oc to frequency, while ohmic 
resistance is independent of frequency. 







BLEGTEICAL ENGINEERING TESTING 


313 


Further, impedance depends on the remaining component 
factor, namely, self-induction, which latter in most cases is due 
to a coiled circuit surrounding iron. Now, the self-induction of 
a circuit depends on the linkage of turns with magnetic field, in¬ 
creasing directly with the latter and with the square of the former. 
Thus it depends on the current, which in turn decides the degree 
of magnetic saturation of the core. The self-induction of the 
armature of an alternator is really only an average of several 
values obtained for different positions of the armature coils 
relatively to the field poles, and it affects the “ wave form ” of the 
voltage generated. Since the self-induction L is directly x to 
the mean permeability (g) of the magnetic circuit, it follows that 
in cases where R is small compared with the Lp, the impedance 
will vary nearly in direct proportion to />t, consequently a curve 
between impedance and current will approximate to the orthodox 
permeability curve of the core. 

On the other hand, with a low resistance winding, the voltage 
absorbed in it, due to the term AR^ is so small that the voltage 
at the terminals is practically that due to self-induction only, 
and hence directly x to the core flux. Thus a curve between 
terminal voltage and current will have the shape of that part of 
the magnetization curve between the origin and “knee,” the 
higher parts of the curve being absent owing to the low degree 
of magnetic saturation used in the cores of A.C. plant. 

Turning now to considerations relative to capacity, the funda¬ 
mental definition of an electrical condenser being that of two 
conductors (called the coatings), separated by an insulator (called 
the dielectric), it follows that on connecting the coatings to a 
source of E.M.F., positive and negative quantities of electricity 
will flow on to them in raising them to the same difference of 
potential as that of the source. The attraction between these 
two quantities sets up a corresponding stress in the dielectric 
and causes them to remain “bound” after the charging source 
is removed. The charge of the condenser is measured in 
coulombs, and is the quantity Q (which = the current in amps X 
time of flow in seconds) necessary to raise the voltage between 
the coatings to the value F of the source. Thus, if C denotes 
the capacity of the condenser, we have Q — CY^ or the capacity 

(7 = — = a constant for any condenser and for all charging 


314 


ELECTRICAL ENGINEERING TESTING 


currents as distinguished from the self-induction of a coiled 
circuit which is not constant, when containing magnetic material, 
but varies with the magnetic saturation of the core, and hence 
with the current. If the source of E.M.F. is an alternating one, 
the state of charge of the condenser will follow exactly the 
change of voltage, reaching a and — maximum, each once 
in every period of the supply. While, therefore, the current 
flowing in an A.C. circuit containing a condenser is actually a 
charge and discharge current alternately^ and does not flow con¬ 
tinuously through, owing to the impassable dielectric insulation, 
an A.C. ammeter placed in the same circuit, by its steady reading 
and inability to follow the rapid pulsations of current, makes 
it appear as if the current really passed through the condenser, 
though it does not do so. 

Again, the internal or ohmic resistance of a self-inductance 
affects the corresponding impedance, while the internal resistance 
of a given condenser has no such effect on the corresponding 
impedance. From the relation already given for the current in 
amperes A = CpV where C — the capacity in micro-farads 
and V = terminal pressure in volts, it will be seen that at the 
smaller pressures of 100 volts or so at about 50 per sec., a 
considerable value of (C) will be needed to give an appreciable 
current. Since, therefore, the capacity available is usually well 
under 110 mfd.s, only a small current will result. In this case, 
care should be taken- that the voltmeter used does not affect 
the voltage across the points between which it is applied, a 
condition fulfilled by the use of an electrostatic voltmeter. 

The tests which immediately follow are arranged to show the 
variation of the quantities indicated with the factors composing 
them, only one of which must be varied at a time in order to 
test its influence on the main quantity. 


(io8) Determination of whether a Resistance 
is truly Non-Inductive at any Frequency 
and Current. 

Introduction. —As in nearly all laboratories there is usually 
a shortage of rheostats, more particularly those of a non-induc- 


ELECTBICAL ENGINEERING TESTING 


315 


tive nature, which are essential in the majority of A.C. tests, the 
present determination is both instructive and useful. 

It is obvious that any inductive resistance must possess some 
ohmic resistance, while a so-called ohmic resistance usually 
exhibits some slight inductiveness. Carbon plate, liquid and 
glow-lamp rheostats are usually taken to be non-inductive for all 
practical purposes, which they are, especially the two first named. 
Since carbon filament lamps are usually employed in lamp 
rheostats in pure parallel combinations, the inductiveness of one 
lamp with its filament of one or more turns is finite though very 
small, and hence that of any combination is still smaller and 
practically nil. 

Liquid rheostats usually consist of two or more metal plates 
dipping into a container of water, the conductivity of which is 
increased to any desired extent by the addition of a little 
common washing soda or aluminium sulphate. Such rheostats 
are undoubtedly less convenient than the carbon or lamp types, 
because, although electrolytic action with A.C. is negligible, they 
froth and alter in resistance considerably with rise of temp¬ 
erature, due to the absorption of the power in them. 

Wire-wound rheostats, whether composed of wire spirals 
wound in a continuous spiral or non-inductively, are usually 
prominent in most test rooms. How far such rheostats, whether 
wound with high-resistance alloys (usually non-magnetic) or with 
iron (which is highly magnetic) are non-inductive, is the object 
of the present investigation. With the former, the self-induc¬ 
tion would be constant for all current densities, but would vary 
with the frequency. Further, the effective or apparent resist¬ 
ance increases for increase of cross-sectional area of wire with 
alternating current, and the self-induction varies as the (number 
of turns) 2 X sectional area of spiral -i- length of spiral. 

Apparatus.—Alternator D, capable of being driven at a wide 
range of speeds, so as to obtain a corresponding range of 
frequency at constant voltage V by varying the exciting circuit 
(not shown) ; Siemens electro-dynamometer, hot wire or other 
A.C. ammeter A unaffected by frequency; electrostatic volt¬ 
meter V; non-inductive wattmeter W; switch >S', and resistance 
It to be tested. 

Observations. —(1) Connect as in Fig. 109, and adjust all the 


316 


ELEGTlilCAL ENGINEERING TESTING 





5* 

0 — 

w 



Iaaaa 

ww 





-'d 


instruments to zero. Then 
start E, seeing that the 
lubricating arrangements feed 
properly. 

(2) By field regulation ad¬ 
just the voltage V across the 
Fig. 109. terminals of i?, that a con¬ 

venient current flows through it as indicated by A. 

(3) Obtain about ten or twelve different speeds of E from the 
smallest to the largest practicable and safe, varying the ex¬ 
citation so as to keep A constant throughout. Note simul¬ 
taneously the readings of A, V, W and speed. 

(4) With the speed, and hence i\\Q frequency adjusted to some 
convenient value to be kept constant, vary the voltage by field 
regulation so as to obtain 8 or 10 different currents through R 
between 0 and maximum safe value, noting the readings of 
A, V, IF, and speed at each, and tabulate your results as 
follows— 


Name . . . Date . . . 

Alternator: Periods per Revolution K = . . Resistance R = . . . ohms. 
Constants: Wattmeter = . . . and A = .. . Nature of Resistance R . 


Speed 

Revs. 

per 

min. 

(A). 

Frequency 

per sec. 

f-FF 

60 

Volts. 

V. 

Scale 

Reading of 

Amps. 

aj_. 

True 

Watts 

iri 

= A'l JF. 

Appnrent 

Watts 

a^F. 

a^^R. 

V 

Cos Q 

- ill 

a^F 

A. 

IF. 













(.5) Plot curves having values of F and as abscissfe in each 

V 

case, and V, cos 0, and — as ordinates. 

Inferences. —State clearly all that you can deduce from your 
experimental results and your curves. 


(109) Measurement of Power Factor in Alter¬ 
nating Current Circuits. 

Introduction. —Alternating-current ammeters and voltmeters 
measure the mean or average value of the current or voltage in 





































ELECTRICAL ENGINEERING TESTING 


317 


an A.C. circuit, and the product of their readings is, therefore, 
the product of two mean values. If the circuit is non-inductive, 
this product of the mean or average values is the true mean or 
average value of the power in watts given to the circuit. If the 
circuit is inductive and possesses self-induction or capacity or 
both, the above product does not give the true mean power, but 
only what is commonly called the volt-amperes of apparent power 
in watts. The true mean or average power in this case is given 
by the mean or average value of the product (amperes X volts) 
in the circuit, for the mean product of two periodic functions 
representing cun'ent and pressure is not equal to the product of 
their mean values. 

Now, the mean value of the product, which thus represents 
the true power in watts, can be measured directly by a watt¬ 
meter, and the ratio 

true power in watts wattmeter reading 
apparent power in watts amps. X volts 
is called the power factor of the circuit, which in practice can 
vary only between the two extreme values 0 and 1. 

This limiting variation, together with the differ¬ 
ence observed between the true and apparent 
power in watts in an inductive circuit, is explained 
by the fact that the current and voltage in such 
a circuit are not in phase, as will be understood 
by a reference to the so-called vector diagram 
(Fig. 110). Let the voltmeter reading be repre¬ 
sented in magnitude (on some convenient scale) Fig. 110. 
by the length of a straight line 0 V and the direction 
of action of the pressure or voltage by the arrow head, i. e. from 
0 to V. 

Thus OF is the voltage vector. Similarly let OA be the 
current vector for the ammeter reading, differing in phase from 
0 F by an angle 

Now OA can be resolved into two component currents at 
riffhi ansdes to one another—the one Oa in line with OF, the 
other (0^) at right angles to it. Then Oa is called the useful 
energy- or load-current^ and Oh the useless-, idle-, or icattless- 
current^ connected solely with the periodic charge and discharge 
of energy in the circuit due to its inductance OaA is therefore 







318 


ELECTRICAL ENGINEERING TESTING 


a triangle of currents for the inductive circuit in which OA 
is the resultant (or ammeter current) of two other currents, 
namely, an energy current Oa and an idle current aA always 
differing in phase by 90®. Then the product of the ammeter 
and voltmeter readings = Off X OF = the apparent watts, the 
wattmeter will give a reading = OF X Oa = the true, or useful, 
watts, while the wattless power will be given by OF X Oh 
watts, which does no ivork in the circuit. 

Thus from the geometry of the figure we have the ratio 

true watts 0 F X O^i Oa 

apparent watts 0 F X Off Off 

= cos (j) = the 'power factor of the circuit. 

Although obtainable in other ways (vide p. 381), this am¬ 
meter, voltmeter, wattmeter method is by far the best and most 
direct one for measuring power factor, and is almost invariably 
employed. 

The evaluation of the power factor cos <j5> in the case of single-, 
two-, and three-phase A.C. inductive circuits, by this direct 
method, is given on p. 388 et seq.^ and in the following test we 
shall restrict ourselves to single-phase circuits. 

Apparatus. —Precisely that detailed for test No. 111. 

Observations. —(l) Connect up as in Fig. 112, levelling and 
adjusting to zero such instruments as need it. The extremities 
of the combination of C and r are the terminals of the 
circuit of which the power factor (P.F.) is required. As, how¬ 
ever, it is sometimes necessary in A.C. testing work to obtain 
either an electrical load at varying P.F. or a variable load at 
constant P.F. with a choker and resistance, it is both useful and 
instructive to determine the effect on the value of the P.F. of 
changing (A) the ohmic resistance, (B) the inductance (whether 
by change in current strength or in disposition of magnetic 
circuit), and (C) the frequency—one at a time. 

(2) A.—Note the readings of all the instruments, for each 
position of the two-way key A, for some eight different values 
of (?•)—the frequency F and current ff being kept constant 
throughout. 

(3) B.—Note the readings of all the instruments, for each 
position of the two-way key K, for some eight different values 
of current ff, covering the range of current utility of the circuit 




ELECmiCAL ENGINEERING TESTING 


319 


or appliance in use—tlie frequency F and resistance r being 
kept constant throughout. 

(4) C.—Note the readings of all the instruments for each 
position of the two-way key /i, for some eight different values 
of frequency F —the resistance r and current being kept constant 
throughout. 

Tabulate all vour results as follows— 


Coil (C)Length = . . . No. of turns = . . . Resistance (ft) = . . . ohms 
Core: Length = . . . Sections = . . . 

Non-Inductive Resistance (?•): Nature. . . . 


Ohmic Resistance 

Current A. 

1 Frequency F. 

For Inductive Circuit XZ 

For Non-Inductive Circuit AT 

Non-Inductive 

r = — 

A 

f-H + 

-S 0^ 

H 11 

CQ 

O 

> 

Apparent Watts 
AV. 

True Watts 

W. 

Power Factor 

W 

cos (}> = 

Volts 

Apparent Watts 
AF^ 

True Watts 

IF,. 

o 

-*-» 

o 

ci 

o 














(5) Plot curves having values of power factor as ordinates, 

with values of Ft in obs. 2; current A in obs. 3; and frequency 
F in obs. 4—respectively as abscissae in each case. 

Inferences. —State clearly all that can be deduced from the 
tables of results and the curves. 

(no) Determination of the Effect of Fre¬ 
quency on the temperature of a given 
Circuit containing Self-Induction and Ohmic 
Resistance only. 

Introduction. —The present test is devised with a view to 
ascertaining whether change of frequency materially alters the 
temperature of any appliance possessing self-induction and 
ohmic resistance, and for the success of the investigation the 
coils of the appliance used should have a low ohmic resistance, 
so that transference of heat due to the term C’^R to the core in 
which any alteration of temperature is to be observed, may be 
as small as possible. 
































320 


ELEGTRIGAL ENGINEERING TESTING 


Apparatus. —Alternator D capable of having its speed varied 

between wide limits, and whose 
voltage can be regulated by the 
exciting circuit (not shown); am¬ 
meter ^ 251); hollow solenoid 

C to test, having a bundle of fine 
soft iron wires, smaller than the 
internal hollow, to allow of a ther¬ 



Fig. 11] 


mometer being inserted as well as 


the bundle. 

Observations. —(1) Connect up as in Fig.Ill,and insert the 
core and thermometer in the solenoid, covering up the ends with 
cotton wool to prevent external cooling effects due to the air, etc. 
Note the temperature when steady. 

(2) Adjust the speed and excitation of I) so that the frequency 
has the lowest value practicable, and the current A some con¬ 
venient value not high enough to heat the coils much. 

(3) The speed and current A being kept constant, take the 
temperature T'‘ on the thermometer at successive noted intervals 
of time (t) from switching on until it remains constant, and 
note also the temperature of the room at intervals. 

(4) Repeat 2 and 3 for the maximum speed allowable, and for 
one intermediate between this and the first-named, the core 
having been allowed to cool down in between each distinct set 
of observations, and the current A being the same. Tabulate 
your results as follows— 


Name . . Date . . . 

Nature of Coil tested . . . Form of Core . . . 

Periods of alternator per Revol. K = . . . 


Temperature of 
Room. 

Temperature of 
Core for Start 

2" C. 

Time from start 

t. 

Speed Revs, 
per min. 

N. 

Frequency 
per sec. 

KN 

~ 60 







(5) Plot a curve for each frequency on the same sheet, having 
temperatures T° C. as ordinates, and (t) minutes as abscissae. 

Inferences. —What can you infer from your experimental 
results and the curves] 

















ELECTRICAL ENGINEERING TESTING 


321 


(ill) Variation of Impedance with {a) Self- 
Induction, (d) Frequency, and (c) Ohmic 
Resistance in Circuits having Self-Induc¬ 
tion and Ohmic Resistance only in Series. 

Introduction. —In this test it will be necessary to vary one 
factor only at a time, keeping the remaining two constant. It 
should also be remembered that, as the coefficient of self- 
induction of any coiled circuit is the number of lines of force 
linked with it when unit current flows through it, any change 
in the current will alter the permeability of the magnetic path 
when this is composed partly or wholly of magnetic material, 
and hence also the self-induction. With an air-core the self- 



induction will be constant for all values of current. The variation 
of impedance and self-induction of an iron-cored solenoid with 
the position of the core has already been investigated in test 
No. 118, and the present determination can be conveniently 
made for fixed positions of the core P (Fig. 123). 

Apparatus.—Source of A.C. supply E, preferably a motor- 
driven alternator, the excitation and speed of which are inde¬ 
pendently variable within wide limits; switch aS" ; frequency 
meter F; ammeter A; voltmeter V; wattmeter W; two-way 
key K; two non-inductive variable rheostats R^ and r {e.g. banks 
of lamps or carbon-plate rheostats); solenoidal choker C with 
movable iron core. 

Note.— is needed (only if the supply F is from town mains) 
for keeping the current A constant, as r is varied. ^ 1^2 
to be taken as the terminals of the impedance. 

Y 







322 


ELEGTBIGAL ENGINEERING TESTING 


Observations. —(a) Impedance with Self-induction at Constant 
Frequency and Ohmic Resistance. 

(1) Connect up as in Fig. 112, levelling and adjusting such 
instruments to zero as need it. Start the alternator with field 
regulator “full in,” and see that the lubricating arrangements 
are working properly. 

Note. —The following tests, Nos. 2 and 3, can be made on 
a public A.C. supply instead, if desired. 

(2) AVith the core P clamped centrally in the coil (7, and the 
speed adjusted to give a frequency F (= no. pairs of poles 
X revs, per min. 60) of 50 per sec., close S, and by field 
regulation obtain some eight different currents on A, rising by 
about equal increments from 0 to the maximum safe value for 
the coil (7, and note the readings of F, A, W and V at each, the 
frequency F and resistance r being constant throughout. 

(3) Readjust the field regulator to “full in,” and with the 
core P removed altogether, repeat 2 for the same constant 
frequency and range of currents. 

{h) Impedance loith Frequency for Constant Self induction and 
Ohmic Resistance. 

(4) Fulfil obs. 1 a})ovej a variable speed alternator now being 
a necessity. 

(5) AVith P clamped centrally, adjust the speed and conse¬ 
quently the frequency F to the lowest convenient value, and the 
current A. (by field regulation) to, say, half the maximum value 
for the coil, to avoid much change of ohmic resistance by 
heating. Note the readings of F, A, W and V at each of some 
eight different values of F between the lowest and highest 
convenient, obtained by speed regulations, the above value of 
current A being kept constant throughout by field regulation. 

(6) Repeat 5 with the core removed altogether, and for the 
same constant current, and tabulate all the results of obs. 2 to 6, 
as shown. 

(c) Impedance loith Ohmic Resistance for constant Self-induction 
and Frequency. 

(7) AVith the core of C clamped centrally and a constant 
frequency F of, say, 50 ^ per sec. adjust the current A to 
about half the maximum safe value for C (to minimize heating) 
and keep it constant throughout (by varying R^ with town 


ELECTRICAL ENGINEERING TESTING 


323 


supp]y for E or by field regulation with an experimental alter¬ 
nator for E) for some eight different values of r between 0 and 
the highest convenient, noting at each the readings of F, A, W 
and F, when the latter is connected by K across and r 

respectively. 

(8) Repeat (7) with the core of C removed altogether for the 
same current and frequency, and tabulate as follows— 


Coil(C): Length =* . . . No. of turns = . . . Eesistance (iJ) = ohms. 
Core: Length = . . . Section = 

Non-inductive Resistance (r): Nature ... p = InF = . . . 


o 


OQ 

O 

0-1 


Voltage across 

Impedance Vj. 

Non-Inductive 

Resistance 

V,. 




<D 

O 

s 

P. 

3 


+ 




Ohmic Resiataiice 

D 

-.j f. 

o ^ 

^ II 

H 





(9) From obs. 2 and 3 plot curves having values of impedance, 
as ordinates, with values of A, Z, and Lp as abscissae in each 
case; also between V and L as ordinates and A as abscissae. 

From obs. 5 and 6 plot curves having values of impedance, as 
ordinates, with values of F and {Lp) as abscissae. 

From obs. 7 and 8 plot curves between impedance as ordinates 
and Ft as abscissae. 

Inferences.—From a careful study of the tables of results and 
forms of curves state clearly all that can be inferred therefrom. 

(ii2) Numerical and Phase Relations between 
the Voltages and between Voltage and 
Current in Circuits containing Capacity 
only and when in Series with Ohmic 
Resistance. 

Introduction.—The numerical relations between the various 
voltages in a circuit such as is now under consideration, seem at 
first sight to be so impossible that it is necessary to consider 
them in relation to phase. This can be done by a reference to 
the vector diagram (Fig. 113). In this, the total or resultant 

































324 


ELEGTRIGAL ENGINEERING TESTING 


voltage^ as indicated on the ammeter A, is set off in magnitude 
and direction = OR. The energy voltage or ohmic drop OG is 
set off at an angle in advance of OD, while GD will be the 
condenser, idle, or reactive voltage in magnitude and direction. 

Comparing this with Fig. 108 for a self-inductive circuit, we 
see that capacity causes the current and its vector OAj^ to lead 
in front of the voltage OD by an angle instead of to lag 
behind as shown in Fig. 108 for self-inductive circuits, both 
diagrams being supposed to rotate about 0 in the direction 
(counter-clockwise). The angles B and G being right angles, lie 
(by geometry) on a semicircle, which is consequently the loci of 
the point of intersection of the energy and reactive voltages 
(which always differ by 90° in phase) between their limiting 



values of resistance R and capacity C respectively, namely, that 
of ^ = maximum with C = 0^ for which = 0, OG = OD, 
and GD = 0 ; and that of ^ = 0 with (7 = maximum, for which 
= 90°, GD = OD, and OG ~ 0. If for the triangle of volt¬ 
ages OGD we divide 'each of the voltages by the current A^, 
the sides will represent the corresponding resistances in circuit, 
while the voltage vectors, as given if divided by these resist¬ 
ances respectively, will give the triangle of currents. The idle 
or wattless current equals A^ sin and the energy current 
equals A^ cos 

Apparatus. —Source of A.C. supply E of, say, constant fre¬ 
quency, such as town mains; switch S ; variable non-inductive 
resistances r and R; ammeter A ; electrostatic voltmeter V ; 
two two-way keys K 1 K 2 1 condenser G. 

Observations.—(1) Connect up as in Fig. 114, levelling and 
adjusting to zero A and V, if necessary. 

(2) With R adjusted to 0 close S, and by varying R obtain 



ELECTRICAL ENGINEERING TESTING 


325 


some six or eight values of current A, rising by about equal 
increments from 0 to the maximum possible, noting the values 
of Fi, Fg, and Fg on F by means of Ki and 



Note. —A convenient form of non-inductive resistance to use 
for R would be an 8 C.P., 16 C.P., and 32 C.P. glow-lamp, each 
of the same voltage as that of the supply, and arranged so as 
to be paralleled in any combination, thus giving seven possible 
different resistances of wide range. Tabulate your results as 
follows— 


Capacity used: Type and Maker . . . Value = . . . mfds. 

Non-Ind. Res. used: Nature . . . Frequency (constant)/= . . . per sec. 


Voltage across 

Amps. 

A. 

Ohmic 

Resistance 

A 

Impedance 

Fs 

a' 

f-i A 

o ^ 

Cu O 

Angle of 
Lead 
</>. 

Supply 

Condenser = 

Idle volts {J jiirfC) 

j 

Non-Inductive 
Resistance = 
energy volts (AR) 
^2- 



1 






(3) Plot curves having values of F^, as ordinates, with Fg and 
C as abscissae respectively. Also between cos </> and impedance, 
as ordinates, with values of R as abscissae. Compare Fg with the 
algebraical sum ( Fj^ + Fg). 

Inferences. —State clearly all that can be deduced from your 
results. 




























326 


ELECTRICAL ENGINEERING TESTING 


(113) Variation of Impedance with (a) 
Capacity, (^) Frequency, and (c) Ohmic 
Resistance in Circuits having Capacity 
and Ohmic Resistance only in Series. 

Introduction. —While the term impedance is applied almost 
universally to denote the apparent resistance of an alternating 
current circuit containing self-induction and ohmic resistance 

only, it is also used here to denote the expression J 
for the apparent resistance of an A.C. circuit having capacity {C) 
and ohmic resistance [R) only, the angular velocity p of the 
current being = 27r X frequency. As therefore it contains three 
variable factors, it will be necessary to vary one only at a time, 
keeping the remaining two constant. 

When C and p are respectively the variables, we may make 
R = 0 and .determine the effect of each on the remaining term, 

= i, called the reactance or reactive resistance of the 

(J2p2 (Jp 

circuit. 

Apparatus. —Source E of A.C. supply, preferably a motor- 
driven alternator, the speed and voltage of which can be varied 



Fig. 115. 


within wide limits; ammeter A', voltmeter F; w^attmeter TF; 
frequency meter jF; switch S', variable capacity C variable 
non-inductive resistance r; two-way key K', 'variable non-in¬ 
ductive rheostat R^ (only needed if E is town mains). 

Observations. — (a) Impedance with Capacity at Constant 
Freqiiency and Ohmic Resistance. 













ELECTRICAL ENGINEERING TESTING 


327 


(1) Connect up as in Fig. 115, levelling and adjusting to zero 
such instruments as need it. Start the alternator with field 
regulator “ full in,” and see that all lubricating arrangements 
in use are working properly. 

(2) With the alternator up to maximum speed, and therefore 

giving maximum frequency F (= No. of pairs of poles X rev. per 
min. -y 60), close S, and adjust G to some six or eight different 
capacities, rising by about equal increments from the smallest to 
largest available, noting the reading of F,’A^ C and of IF and V 
on at constant frequency F and resistance (r). 

(3) With r = 0 repeat obs. 2 for the same values of (7, noting 
IF and V on in addition to F, A^ (7, for the same constant 
frequency. 

(d) Impedance with Frequency for Constant Capacity and Ohmic 
Resistance. 

(4) A variable speed alternator being available, make C a 
maximum and give r some convenient fixed value. Now read 
all the instruments for both positions of K at each of some 
six or eight frequencies differing by about equal amounts 
between the maximum and minimum values obtainable by speed 
regulation. 

(5) With r = 0 repeat obs. 4 for the same constant value of 
C and range of frequencies. 

(c) Impedance ivith Ohmic Resistance for Constant Capacity 
and Frequency. 

(6) Give G and the frequency their maximum possible values, 
and take the readings of all the instruments with K on studs 
1 and 2 for some six or eight values of 7*, differing by about 
equal amounts between maximum and zero values. 

Tabulate all your results as follows (where p = 27r/)— 


Frequency /. 

Amps. A. 

Capacity C. 

2 

11 

<n ¥ 

a - ^ 

p* i-i e-i 

Volts across 
non-Ind. Res. 
r= V^. 

Ohmic Res. r. 

® CC 1 

y c "T 

G 5 e* 

ce 7: 

2|^> 

!l 

Reactance 

1/Cp ohms. 

1 

CO 

CO lit 

0 s 

03 ^ 

u s 

y 

1 " 
oc ^ 

Wattmeter across 
r = Walts. 

0 

0 

c 4 II 

CO ** 

- 

CO 

4 ^ 

c 3 

II 

Pi 

<3 

ft, ^ 

^ II 

u 

Angle of Lag 




1 



1 









































328 


ELEOmiGAl ENGINEEMJNG TESTING 


(7) From obs. 2 and 3 plot curves having values of impedance 

reactance J- and amps. A as ordinates with values of 
\A) Cp 

capacity C as abscissae. 

From obs, 4 and 5 plot and A as ordinates with 

A Cp 

frequency (/) as abscissae, and from obs. 6 plot ^ as ordinates 

with ohmic resistance (r) as abscissae. 

Inferences.—State clearly all that can be deduced from the 
above results. 


(114) Variation of Impedance with (a) Self- 
induction, ((^) Capacity, p) Ohmic Re¬ 
sistance, (d) Frequency in Circuits having 
Characteristics a, b and c in Series. 

Introduction.—It has been stated that self-induction L causes 
the current to lag behind the voltage, while capacity G causes it 
to lead in front of the voltage. A circuit possessing both L and 
C may therefore cause the current to lag behind, lead in front of, 
or be in phase with, the voltage, depending on the relative mag¬ 



nitudes of L and G for a given ohmic resistance and frequency. 
Each of the two last named will in turn affect such phase rela¬ 
tions, and hence in the present investigation we have four possible 
variables composing the impedaii'^e or “ apparent ” resistance 
of the circuit, only one of which must be varied at a time with 
the remaining three kept constant. 





ELECTEIGAL ENGINEEKING TESTING 


329 


The phase difference between current and voltage will be less 
than that which would be caused by the same value of either 
L or C alone, and, as stated above, may even be zero. The 
above remarks will be better understood by a reference to 
Figs. 116 and 117. 

Let OA be a vector representing, in magnitude and direction, 
the current A. Set off Oh = the voltage Fjj( = Alt), which is 
an energy or useful voltage in phase with A, along OA. 

With centre {b) and radius ha = Vl describe an arc of a circle, 
and with centre (0) and radius Oa = Vl + r describe an arc 



of a circle (Fig. 116), intersecting the other arc in the point (a). 
From (a) drop a perpendicular to OA meeting it in {d) and 

produce it to (e) so that cZe = Vc — ~ the condenser voltage. 

{jp 

This will be 180° out of phase, i. e. in direct opposition to the 
voltage overcoming the self-induced idle voltage {ea) = LpA. 
Consequently the nett or effective idle voltage of the circuit 

da = a{^p — where = the effective or nett in¬ 

ductance of the whole circuit FQ, and is of a self-inductive 
nature causing an effective angle of lag d in the circuit. In 
the triangle Ode, Ar = 0, since the condenser 0 is not con¬ 
sidered to have any ohmic resistance ?• like the self-inductance 
Z has. Therefore Od = the energy voltage AF for the portion 
FN, Od is therefore also the useful or load component of the 
total voltage of the circuit FQ. 

A most important deduction, affecting the calculation of the 
rise of pressure in cables and sometimes the breakdown of their 
insulation, now follows, namely, if the reactances of the self- 















330 


ELECTRICAL ENGINEERING TESTING 



p = 2irf the supply frequency, the idle voltages of L and 

C will be equal and opposite in sign, and each may have much 
greater values than that of the supply. This condition in such a 
combination, shown in Fig. 117, is called resonance. 



of the circuit is negligible is the periodicity of the supply equal 
to the 7iatural periodicity of oscillation. The natural period of the 


circuit j. = 277-y /LC seconds. The periodicity giving maximum 


resonance in a circuit of appreciable ohmic resistance R is 



which is not the natural periodicity of 


oscillation of the circuit. 

Either of the above values of the critical frequency (/) giving 
maximum resonance is usually much greater than that of the 
supply voltage. 

Apparatus. —Source of A.C. supply E, preferably a motor- 
driven alternator having a wide range of speed and excitation; 
ammeter A; wattmeter IV; switch S; voltmeter V; and two 
three-way keys; a capacity C, self-induction L, and ohmic 
resistance ii, each capable of variation; frequency meter (/). 

Observations. — [a) Impedmice with Self-induction at Constant 
F^'eqiiency, Capacity and Resistance, 

(1) Connect up as in Fig. 117, levelling and adjusting to zero 
such instruments as need it. Start the alternator with field 
regulator “ full in,” and see that the lubricating arrangements 
are working properly. 

(2) With the self-inducticn Z, resistance R, and capacity C 
adjusted to convenient values, and the speed to give a frequency 
F {= No. of pairs of poles X rev. per min. 60) of 50 per sec., 
close S and by field regulation obtain some eight different currents 
on A (and hence values of L) rising by about equal increments 
from 0 to the maximum safe value, noting the values of F, A, V 
and C at each, F, R and C being constant throughout. 









ELECTRICAL ENGINEERING TESTING 


331 


(6) Impedance with Capacity at Constant Frequency^ Felf- 
induction and Resistance. 

(3) Repeat obs. 2 for some eight different values of capacity 
C between 0 and the maximum possible, F, R and A (i. e. L) 
being constant throughout. 

(c) Impedance with Ohmic Resistance at Constant Frequency^ 
Capacity and Self-induction. 

(4) Repeat obs. 2 for some eight different values of resist¬ 
ance R between 0 and the maximum possible, F^ C and A 
(i. e. L) being constant throughout. 

[d) Impedance with Frequency at Constant Self-inductiont 
Capacity and Resistance. 

(5) Repeat obs. 2 for some eight different frequencies between 
the minimum and maximum values possible, C, R and A (i. e. L) 
being kept constant. 

Tabulate all your results as follows—• 


Self-Induction used : Nature or type . . . Ohmic Res. r = . . . ohms. . . . 
Ohmic Res. (iJ) used: Nature or type . . . 

Capacity (C) used : Type ... p = SttF. 



(6) Plot curves having values of impedance as ordinates with 
each of the variables A (or Z), (7, Rt, F in obs. 2-5 as 
abscissae on the same curve-sheet. 

Inferences.—State clearly all that can be deduced from the 

results of the test. 

jjote. _The numerical and phase relations between the 

voltages across (7, R and Z, and the combinations of these can 



















































332 


ELEGTBIGAL ENGINEERING TESTING 


be studied in the above table with advantage and are highly 
instructive. From them the student should draw to scale the 
diagram shown in Fig. 116 above, for, say, two extreme values 
of the overall voltage Vcrli see how the angle of phase 
difference 6 compares with that calculated in the above 
table. 


(115) Numerical and Phase Relations be¬ 
tween Main and Branch Currents in 
Circuits containing Ohmic Resistance in 
Parallel with either Self-induction or 
Capacity. 

Introduction. —The determination of the above relations 
between the main and branch currents in a circuit comprising 
ohmic resistance and self-induction in parallel is effected in 
detail in test No. 134, which should be done for the present 
test. 

The numerical relations are at once seen in the table of 
results, while the phase relations are best seen from the diagram 
(Fig. 142) constructed for any particular set of simultaneous 
currents. It will be obvious that the relations will differ 
according to whether the self-induction branch PQ possesses 
appreciable ohmic resistance or practically none. Fig 141 presumes 
the former condition, but if the latter obtains, then the current 
A-^ in the non-inductive branch, being in phase with the voltage, 
will be given by OC, while that in the inductive branch A^ 
(having no resistance) lags just 90° hehinl the voltage, and will 
now be given by Ca (perpendicular to 0(7), instead of by ha as 
in Fig. 142. 

The present test should also be operated with capacity C 
substituted for the self-induction shown, when the student should 
have no difficulty in modifying both the tabular form of entry 
and the vector diagram to suit the new condition of capacity 
in parallel with ohmic resistance. If no resistance is purposely 
added to the condenser branch, the current in this will lead 
just 90° in advance of the voltage. Thus, the main or resultant 
current will be given by the diagonal of a parallelogram, the 


ELECTRICAL ENGINEERING TESTING 333 

sides of which will be at right angles and represent the branch 
currents. 


(ii6) Variation of Impedance and Phase 
Relations between the Currents in a 
Circuit containing Capacity and Self- 
Induction in Parallel. 

Introduction. —The combination of self-induction in parallel 
with capacity is an extremely important one, and has some 
interesting and highly useful aj)plications in electrical engineer¬ 
ing which will be mentioned later. Referring to Figs. 118 





and 119. Let OE = the supply voltage V, and if the ammeter 
Ac and switch So add a negligibly small resistance to the con¬ 
denser branch, the current in this branch will = CVp and 


tan Qc~- 7 t~t— ^ 1 whence Aq will make an angle of phase 
Cpi'c 

difference POE — Be — 90° in advance of V. 

Similarly, if the ohmic resistance of the self-inductive branch 
is negligibly small, the max. current Aj^ in this branch 

will = and tan Bl — — = i whence A r will lag behind F by 
Lp r ^ j 

an angle MOE Bl~ 90°. OP and OM are therefore the max. 

values of the respective branch currents. If, however, the self- 

ind. branch possesses a resistance (r) ohms in addition to self- 

F 

ind. Z, its current will b.e given by Ai =^— / —v-^ = 0^ 


\JL^p"^ -j“ 

lagging behind V by an angle Bl — QOE, such that tan Bl — 


Lp 


r 









334 


ELEGTRIGAL ENGINEERING TESTING 


(less than before). The total current A being the resulta-nt OR 
of OF and OQ and making an angle ROE = 6 (seen to be one 
of advance in this case) with the voltage V. 

The semi-circles ORP and OQAI are the loci of the vectors 
representing the branch currents A a and A x, and it will be seen 
that the smaller the resistance (r), the nearer will Q approach i)/, 
and the smaller will be the resultant or main current OR (= A) 
from the supply and the more nearly will it be in phase with 


OE (= V). 




r s, 




1 


w 


-©- 




3 


Fig. 119. 


Thus at the limit, will be very small, Q very close 

to M, Ai, nearly = Ac and nearly 180° out of phase with it, and 
OR very small and nearly in phase with V. Under these condi¬ 
tions current resonance is said to prevail in distinction to pressure 
resonance explained on p. 330 for series circuits. With current 
resonance in such a parallel circuit, the local current circulating 
in the loop may be many times greater than the main supply 
current A —a condition obtained when the idle or wattless 
components of the branch currents are practically equal, but of 
opposite, sign Avhen the wattless or idle component of the main 
current A will be less than that of either branch. Equal and 
opposite wattless currents in the branches will be obtained when 

lICp __ Lp 
\lC‘^p'^r^ L‘^p‘^-\-r‘^ 

but as explained for series circuits this condition can only truly 
be called resonance when both r^ and r are negligibly small. 

In practice we see capacity used for starting up alternating 
current motors; for nullifying the effects of the idle currents 
in a distributing system, thereby raising the power factor, and 
so increasing both the efficiency and economy of operation. The 













ELEGTIUCAL ENGINEERING TESTING 


335 


capacity effect in this case is produced by an over-excited 
synchronous motor connected to the same supply. 

: Apparatus. —Source E of A.C. supply preferably a motor-dri\^en 
alternator variable in speed and excitation within wide limits; 
frequency meter (/); voltmeter F; ^vattmeter TF; ammeters 
Aq and Al] switches S, Sc and S^', variable capacity C ; variable 
self-induction L of ohmic resistance (r). 

Observations. — [a) Impedance with SelfAnduction at Constant 
/, C and r. 

(1) Connect up as in Fig. 119, levelling and adjusting to zero 
such instruments as need it. Start the alternator with field 
regulator “full in,” and see that the lubricating arrangements 
are working properly. 

(2) With f, G and r (if alterable) adjusted to convenient 
values, close S, and then Sq only, taking the readings of all the 
instruments. 

(3) With /, C and r as in obs. 2, close S^ and then S^ only 
and take the readings again. 

(4) With fy C and r again the same, close all 3 switches and 
by field regulations obtain some 8 dififerent currents on A^ (and 
hence values of L) rising by about equal increments up to a safe 
max. value, noting the readings of all instruments. 

(6) Impedance with Capacity at Constant f A^ (i. e. L) and r. 

(5) Bepeat obs. 4 for some 8 different values of capacity C at 
constant fy r and Ai (i. e. L). 

(c) Impedance with Ohmic Resistance (r) at Constant fy A^ (i. e. 
L) and G. 

(6) Repeat obs. 4 for several values of (r) if this is variable at 
constant fy L and G. 

(d) Impedance with Frequency (/) at Constant A^ (i. e. Z), C 
and r. 

(7) Bepeat obs. 4 for some 8 different values of frequency (f) 

at constant Al (i. e. Z), G and r. 

(8) By varying (7, Z, /, find the minimum value of A obtain¬ 
able, noting the readings of all instruments at this, and tabulate 
all your results as follows— 


336 


ELECTRICAL ENGINEERING TESTING 


Frequency (/). 

Voltage V. 

Wattmeter Heading. 

True Watts W. 

Amps. A. 

6 

W 

Ph 

a 

< 

H 

& 

a 

< 

Apparent Watts A F. 

Power Factor 

cos e = WjA V. 

Angle of Phase Diff. d. 

* « 

+ 

t ei 

> 

II 

CO 

O 

O 


a. 

o 

II 

o 

• rH 

o 

CC 

Ph 

a 

Self-Ind. 

V ^ 

6 

Sr 

O 

P 

P 

Pi 

s 

M 

Sr 

o 

o 

p 

C3 

73 

03 

Pi 

s 

Impedance V/A. 


















Inferences. —State clearly all the inferences which can be 
deduced from the above results. 


( 1 17) Determination of the Load and Watt¬ 
less Currents in an Inductive Alternating 
Current Circuit. 

Introduction. —While the present investigation is bound up 
with that of power factor, considered in test No. 109, p. 316, the 
whole subject has such a vastly important bearing on the 
economical and efficient generation, transformation, and dis¬ 
tribution of electrical energy, that a further consideration of it 
will be an advantage. 


E a 



It is well known that of the power in watts, given by the 
product (amperes X volts) ‘‘apparently” supplied to an inductive 
A.C. circuit or one containing self-inductance, or capacity, or 
both, only a portion constitutes an actual or useful load or 
power and does useful work in the circuit, while the other 
portion represents no load at all, and is said to be wattless or 
idle power, doing no work in the circuit. 













































BLEGTEIGAL ENGINEERING TESTING 


337 


riie useful power is given by the product of that portion of 
the current and voltage in phase with each other, and is usually 
called the true power^ while the wattless or idle power is given 
by the product of those portions of the current and voltage 
which are in quadrature, as it is termed, i. e. differ in phase by 
90 or a quarter period, the average value of the latter product 
being always zero. The useful and wattless powers are each 
given by a product of amperes X volts, and may be arrived at in 
either of two ways as follows : let a voltage OE and a current 
OA differ in phase by an angle Resolve OA into two com” 
ponents, one Oa along and in phase with OE, the other Oai 
perpendicular to it. Then OaAai is a rectangle, and the corner 



aj will lie on a semi-circle OcijA dra^vn on OA as diameter. 

We now have the true power = OE X Oa, the wattless powder 

= OE X Oaj, and the apparent power = OE X OA. Oa^A is, 

therefore, the triangle of currents of which OA is the total or 

resultant or ammeter current, ajA the load or useful current, 

and Oaj the idle or wattless current, always perpendicular to ajA. 

rri HI , OE X Oa ajA . 

ihe power factor cos 6 ^ p-j-, - 77-7 =5 -tc-t- = cos OAar. 

^ ^ OE X OA OA ^ 


Again, resolve OE into two components—one OA along and in 
phase with the current vector OA, the other OEj perpendicular 
to it. Then OEjEA is a rectangle and the corner A will lie in 
a semi-circle OAE drawn on OE as diameter. AVe now have the 
true power = OA X OA, the wattless power = OA X OEj. OAE 
is therefore the triangle of voltages of which OE is the total 
or resultant or voltmeter voltage, OA in phase with the current, 
the load or useful or energy voltage, and OEi the idle or wattless 
voltage always perpendicular to OA. The power factor cos ^ = 


OA X OA 
OA X OE 


— cos AOE. 


z 






338 


ELEOmiGAL ENGINEERING TESTING 


Now, the wattless powers in the two cases are OE X Oaj and 
OA X OEj respectively, which are equal, since the areas of the 
rectangles {OE X Oaj) and {OA X OEj) are equal. Mathemati¬ 
cally, therefore, it is immaterial from which point of view the 
matter is treated, as both lead to the same result, namely— 

True power = total voltage X useful current = V X A cos 
,, „ = total current X voltage = A X V cos cf). 

In practice, however, it is more convenient and general to 
consider the total current A to be made up of two components, 
respectively A cos (f> in phase with, and A sin cj) in quadrature 
with, the voltage, and termed the energy, useful, or load current 
and the idle or wattless current. These are related geometrically, 
as seen in Fig. 121, by the equation 

(Total current)^ = (useful current)^ (wattless current)^ 


R 



Fig. 122. 

from which the wattless current measurement of the present test 
is deduced. This can be made with the two possible circuit 
conditions, namely, self-induction with ohmic resistance, for which 
the total current lags in phase behind the load current by an 
angle cf}, and capacity with ohmic resistance, for which the load 
current lags in pliase behind the total current by an angle cf}, 
i. e. the total current leads in front of the load current. This is 
shown in the single diagram, Fig. 121, though more commonly by 
two separate ones split along the line OA. 

Apparatus. —Source E of alternating current; voltmeter F; 
ammeter A ; wattmeter W ; variable non-ind. resistance R ; 
capacity G ; self-induction L. 

Observations.— (1) Connect up as in Fig. 122, levelling and 
adjusting to zero such instruments as need it. 

(2) With G only connected in circuit, note the readings of V, 
W, and A for some five or six values of current A between 0 
and the maximum possible by varying R. 










ELECTRICAL ENGINEERING TESTING 


339 


(3) With L only connected in circuit, repeat obs. 2 and 
tabulate as follows— 


Nature 

of 

Circuit. 

Voltage 

V. 

Watts 

W, 

Currents. 

Power- 

Factor 

cos 

= WfAV. 

Angle of 
Phase Diff. 

Total (OA) 
= A. 

Load 
= JF/F. 

Wattless 

= Va^-( w/ V)\ 









(4) Check one or more of the tabular readings by diagram, 
such as Fig. 121. 

Inferences.—State all that can be deduced from the results 
of the test. 

(ii8) Variation of Impedance, Reactance and 
Self-Induction with Position of Movable 
Core in Solenoidal Choker. 

Introduction.—This test is intended to show the principle 
underlying the action of the so-called dimmer, which is so 
commonly used now in theatres and picture-halls for raising and 


E 



c 


Fig. 123 


lowering the lighting when the supply thereto is alternating 
current, also the range of regulation of a choking coil for working 
on arc lamp circuits of different voltages. It has the great 
advantage over a variable “line” resistance of preserving com¬ 
pletely the electrical continuity of the circuit, while introducing 



































340 


ELECTRICAL ENGINEERING TESTING 


a back E.M.F. of self-induction to the supply depending on the 
position of the movable core. 

Apparatus.—Source of A.C. supply A, preferably an experi¬ 
mental motor-driven alternator, the excitation and speed of 
which are independently variable within wide limits; switch S; 
frequency meter F; ammeter A ; voltmeter V; wattmeter W ; 
and the movable core solenoidal choker C. 

Observations.—(1) Connect up as shown in Fig. 123, levelling 
and adjusting such instruments to zero as need it. Ensure 
that the lubricating arrangements are working properly on 
starting up. 

(2) With the field regulator of F full in and the machine 
supplying constant frequency F, close S and adjust the alternator 
field excitation so as to give the max. safe current A through G 
with the centres of P and C coinciding (i. e. Z) = 0), then note 
the readings of Z", A, W and V. 

(3) Take the readings of F, A, IF, V and P with the same 
constant values of F and A for each of a series of clamped 
positions of P between D =z 0 and Z> = full length of P, with P 
finally removed to a distance. 

(4) Kepeat obs. 2 and 3 for the same constant frequency F, 
but with V now maintained constant^ at such a value as will 
prevent the current rising above the max. safe value when P 
is removed to a distance, and tabulate all your results as 
follows— 


Choker coll: Length «• . . . No. of turns *3 . . . Res. iZ =» . . , 
Core : Length =* . . . Cross Section = . . . 


- 

Frequency F. 

Amps. A. 

Zfl 

Distance 2>. 

Wattmeter Reading. 

True Watts JF. 

Apparent Watts 

AV. 

Power Factor 

COS0=J1. 

AV 

Angle of Lag 6°. 

® 4- 
o 

'il5 

S n 

S 0 

Sr 

® 1 

C n 

II 

a. 

M 

• 

£ 1 

4-1 

"S U. 1-^ 

OQ 

s-l 

® >-i 1 a, 

6 II 














(5) Plot curves for obs. 2 and 3 having values of I) as 
abscissae with values of F, IF, cos 0, F/yf, Lp and L as ordinates 
respectively, and for obs. 4 having values of P> as abscisste with 
values of A, IF, cos 0, F/^, Lp and L as ordinates respectively. 




























ELECTRICAL ENGINEERING TESTING 


341 


Inferences.^ —State all you can deduce from the table of results 
and curves. 

(119) Effect of Length of Air Gap in a Closed 
Magnetic Circuit on Impedance, Re¬ 
actance, Self - Induction, Current and 
Power. 

Introduction. —The present test is a very important one, 
inasmuch that it is a direct proof of fundamental theory, and 
has an important bearing on the use and range of regulation of 
all kinds of choking or reactance coils for adjusting the current 
in arc lamp circuits at different voltages, while at the same time 
emphasizing the relative merits of “ closed ” and “ open ” mag¬ 
netic circuits. The factors of an alternating current supply 
being voltage, current, and frequency, with the last named usually 



Fig. 124. 

constant, it follows that the present investigation can be carried 
out in at least two ways, namely— 

(а) With constant current and varying voltage at constant 

frequency. 

(б) With constant voltage and varying current at constant 
frequency. 

In the former method of supply, and with a series wound and 
connected choker, any effects observed by varying air-gap must 
be due to this alone. In the latter method, owing to change of 
current strength (in all but saturated magnetic circuits) causing 
a change of magnetic flux and induction density, any effects 
otherwise due to change in length of air-gap may be seriously 
vitiated. 

Apparatus. —Experimental magnetic circuit with adjustable 
































342 


ELECTRICAL ENGINEERING TESTING 


air-gap; switch JS; frequency meter F; ammeter A; voltmeter 
F; wattmeter W; source of A.C. supply preferably from an 
experimental motor-driven alternator, the voltage and speed 
{i. e. frequency) of which are independently variable within wide 
limits. 

Observations. —(1) Connect up as in Fig. 124, levelling and 
adjusting to zero such instruments as need it. Now start the 
motor-alternator, observing that the lubricating arrangements 
are working properly. 

Note. —If a constant voltage and frequency town supply is 
used, a suitable non-inductive resistance must be connected in 
series with one of the mains on the supply side of JF for regulating 
the current. 

(2) Remove all the non-magnetic distancing strips D and 
clamp the laminated iron keeper K down on to the poles by 
means of the wing-nut clamp (not shown in Fig. 124). With the 
field-regulating resistance of the alternator full in, close S and 
obtain a frequency of 50 or 60 per sec. on Fy to be kept 
constant by driving the alternator at the requisite constant speedy 
where the frequency (/) = No. of pairs of poles X revs, per 
min. 60. 

(3) Raise the A.C. voltage by field regulation to the maximum 
value possible, so long as the current produced does not exceed 
the safe maximum for the choker winding. Then note the 
readings of F, A, W, F, and that the air-gap is zero. 

(4) Unclamp K, and carefully raise it just sufficiently only to 
slide one distance strip D in between it and the poles, and 
re-clamp K. 

Now, lower the voltage (by field regulation) until A has the 
same value as before—the frequency being also the same. Then 
read F, A, TF'and F. 

(5) Repeat (4) for about ten different air-gaps, increasing by 
one distance strip at a time, and finally with K removed alto¬ 
gether, i. e. air-gap = max. 

(6) Employing supply condition (b), mentioned in the intro¬ 
duction above, with K removed altogether, adjust the field 
regulator of the alternator so as to give such a voltage as will 
send the max. safe current through the choker winding at the 
same frequency as before. Now note the values of F, A, W and 


ELEGTRIGAL ENGINEERING TESTING 


343 


F, and that the air-gap = max. This voltage and frequency 
is to be kept constant in future. 

(7) Next take the readings of Fj A, JV and V for each of a 
series of air-gaps, ranging from that given by all the distance 
strips clamped together between /i and the poles to none in at 
all, by one at a time, and tabulate all your results as follows— 


Form of Inductive Circuit tested . . . 

Section of: Core = ... Yoke = . . . Keeper = . . . 

Distance Strips eacli = . . . Thick : Resistance of Choker winding R = . . . ohm. 



(8) Plot curves from obs. 3-5 having values of D as abscissae 
with F, TF, cos 0, V/A and L as ordinates respectively. Also 
from obs. 6 and 7 plot curves having D as abscissae with A, TF, 
cos F IA and L as ordinates respectively. 

Inferences.—From a careful study of the above table and 
curves state clearly all that can be deduced. 


(i2o) Investigation of Mutual Inductive 
Effects due to the Relative Positions 
of Two Coiled Circuits. 

• Introduction.—The object of this investigation is to find out 
to what extent, and in what way, two neighbouring electro¬ 
magnetic fields may react on one another when in different 
relative positions. 

Qualitative and quantitative results are obtainable which are 
both interesting and instructive, in view of how little the 
average student realizes the possibilities of interaction between 
neighbouring magnetic fields and apparatus with the prejudicial 
effects often resulting. For the investigation, two solenoidal 
movable iron core choking coils (preferably similar in all respects) 
may be used, connected in series. 






























344 


ELECTRICAL ENGINEERING TESTING 


Apparatus. —Two similar chokers; ammeter A ; voltmeter V ; 
wattmeter W ; frequency meter F; switch S; non-inductive re¬ 
sistance R (such as a bank of lamps) for regulating the current, 
if the supply F is from the town. If from an experimental 
motor-driven alternator, R can be omitted and A adjusted by 
field regulation on the alternator. 

Observations. —(1) Connect up as in Fig. 125, where 
are the terminal extremities of the two coils connected per¬ 
manently in series. Level and adjust to zero such instruments 
as need it. 



(2) With coils touching side by side in arrangment * 

{i. e. = minimum) and cores clamped centrally, take the 
readings of F^ IF, F, A and for constant full-load current A 
and frequency 50 for each of a series of values of distance 

up to a convenient maximum, the coils being parallel at 
each. 

(3) Take a copy of an iron filing diagram for the position 
= a min. 

(4) Repeat obs. 2 and 3 with one coil reversed or turned 
through 180°. 

(5) Repeat obs. 2 to 4 for the position of coils shown at 
(i.e. with magnetic axes of cores perpendicular) at different 
distances D^. 

(6) Repeat obs. 2 to 4 for the position of coils shown at C^C 
{i.e. axes in line) at different distances ilg, and tabulate as 
follows— 




























ELECTSICAL ENGINEERING TESTING 


315 


Coil Cl used : Length. No. of turns * . . . Res. R =« . 

Core length =* . . . Cross section = . . . 

Coil Cg used : Length ** . . . No. of turns = . . . Res. ij ^ , 

Core length =» . . . Cross section =s . . . 


Relative 
Positions 
of Coils.; 

Distances 
Pi. P2 
and 1)3. 

Frequency 

F 

(Constant) 

Current 

A 

(Constant) 

Volts 

V. 

True 

Watts 

IF. 

Apparent 
Watts 
Ax F. 

Impedance 

FJA. 

Power 

Factor 

COS<j) = 

JF/AF. 











(7) In addition to the copies of the respective iron filing 
diagrams, plot the following curves having values of distance D 

y 

as abscissae, with volts V and impedance ^ respectively, as 


ordinates in each case. 

Inferences.-— From the table of results, diagrams and curves 
state all that can be deduced. 


(i2i) Measurement of Magnetic Permeability 

by the Permeameter. 


Preliminary. —The following method, devised by Prof. S. P. 
Thompson, is a simple and convenient workshop one for rapidly 
measuring the magnetic permeability (ju,) of any material. It 
is quite distinct from the ballistic, direct magnetometric, or 
optical methods of measuring (/x), and is based upon the law of 
magnetic traction, viz. that the tractive force over a given area 
of contact is proportional to the square of the magnetic flux 
through the junction. This and all other traction methods are 
not capable of giving very accurate measurements of (/x), for 
both the tensile stress and the place chosen for contact between 
specimen and block may affect the results somewhat, as in the 
latter case the distribution of the induction is not very uniform 
at this point. JlSTow, since in the permeameter the magnetizing 
coils remain fixed, the pull on the specimen core will be due to 
- H) lines, where B = induction per sq. cm. through the 
junction, and i/" = magnetizing force producing it. If S= 
sectional area of the junction in sq. cms. the force of attraction 
between core and block, i. e. VxAl {P) = {B — dynes = 


{B - {Sir X 453-6 x 981) lbs.. •. = 1317 x 

O.G.S. lines. Where P = pull in lbs. to detach. 



P 

S{s(\. in.) 


+ // 























346 


ELEGTBICAL ENGINEERING TESTING 


If the magnetizing coil consists of T turns, carrying a current 


A amps., and its length between ends = 1. 


Then //= 


^irAT 
10 ^ 


^KA, 


in C.G.S. measure. (The constant K^i-irT -j-10 /.) 

An advantage with this method of measuring permeability is 
that the specimen of the material to be tested is in the form of 
a simple straight rod of circular cross section, and in this form 
it can generally be very easily obtained. Further, owing to no 
delicate ballistic galvanometer being used in the method, the 
test can be performed even in fairly close proximity to dynamos 
or near other magnetic fields without especially vitiating the 
results. The permeameter illustrated is fitted with a slip coil 
C G for measuring ^ ballistically if desired. For a more detailed 
description of the instrument vide p. 615. 

Apparatus. —Permeameter (Fig. 280); Salter’s spring balance; 
ammeter A ; rheostat rg (p. 599); battery h ; switch ; Pohl’s 

commutator or reversing 
switch S (p. 584); specimen 
or rod R to be tested. 

Tests, —(1) Connect up as 
indicated in Fig, 126, omitting 
the ballistic circuit shown at 
the lower part of the Fig. 
Insert a specimen in the coil 
after having cleaned the end 
and demagnetized it. Then 
attach the spring balance by 
means of its double hook to 
the pin in the present 
case. 

(2) With 9*2 full in, adjust 
the current to a small value 
(say 0 02 amp.), and note the 
force P lbs. required to de¬ 
tach. This should be re¬ 
peated two or three times, and the mean noted. 

(3) Repeat 2 for about fifteen different currents up to the 
maximum (about 7 amps.), rising by such amounts as will give 
about equal increments of pull on the balance. 
































ELECTRICAL ENGINEERING TESTING 347 

(4) Repeat 2 and 3 for different specimens^ and tabulate as 
follows— 


Name . . , Date . . . 


Specimen tested. 

Magnetizing. 

Pull 

Plbs. 

Induction 

B. 

Permeability 

B 


Nature. 

Diameter 
d (ins.) 

Section (S) 

Tfd'^ . 

= -j-Sq.in. 

Current 
A amp. 

Force 

H=KA. 











(5) Plot two curves, one having H as abscissas and B as 
ordinates, the other having B as abscissas and as ordinates. 


(122) Measurement of Magnetic Permeability 
(by Hopkinson Permeameter). 

Introduction.—The present test is very similar to the pre¬ 
ceding one (No. 121), except that a slightly different form of 



permeameter, devised by Professor Hopkinson, is provided. This 
instrument consists (as seen in Fig. 127) of a heavy wrought- 
iron yoke with two magnetizing coils, one having a fixed core 
and the other a movable one. On the movable core is a small 
coil of wire which, when the core is withdrawn, is jerked up by 
a spring; this coil is connected to a ballistic galvanometer, and 
the throw is proportional to the lines in the core; by this means 
cores made of various samples may be compared for perme¬ 
ability. The apparatus and connections, except for the differ- 









































































































































































































348 


ELMGTEIGAL ENGINEERING TESTING 


ence in the actual form of permeameter, and the addition of a 
ballistic galvanometer G, key I{, adjustable standard known 
resistance (r), and earth inductor E, or preferably a standard 
solenoid, are precisely those indicated in Fig. 126, and the 
student should read the introductory remarks of test No. 121. 

The evaluation of the throws on the ballistic galvanometer G 
is exactly as given in the introduction to test No. 78, and 
therefore need not here be repeated. 

Observations. —These consist in taking the galvanometer first 
throw at the moment when the small slip-coil springs out, for 
each of a series of exciting currents ranging from 0 to the 
safe maximum permissible, and tabulating all the readings and 
evaluated results in a convenient form. 

Note.—The first throw should be repeated,/or each excitation^ 
by replacing the slip-coil two or three times, when the mean 
throw only at each current should be tabulated. Further, a 
preliminary trial must be made, before taking the above series 
of observations, by adjusting the resistance (r) until maximum 
permissible exciting current gives a mean first throw, not ex¬ 
ceeding full-scale deflection. Lastly, the increments of current 
in the series must be smaller during that part of the range 
where the magnitude of the mean throws appears to be differing 
considerably. 

A curve should be plotted between values of induction density 
B found, as ordinates, with values of magnetizing force II as 
abscissae, and also one between permeability p, as ordinates, 
and values of B as abscissse. 


(123) Measurement of Magnetic Hysteresis 
and Eddy or Foucault Currents in Samples 
of Magnetic Material. (By Single Phase 
Alternating Currents.) 

Introduction. —The determination of magnetic hysteresis in 
magnetic materials by some of the most important methods is 
given in considerable detail in Practical Electrical Testing by the 
author, and the reader is referred to this book for further par¬ 
ticulars of these tests. It is of course well known that an iron 


ELEGTRIGAL ENGINEERING TESTING 


349 


core, magnetized by an alternating current of electricity, is the 
seat of two distinct losses of power, (1) from hysteresis, and (2) 
from eddy currents generated in the transverse section of the 
conductor. 

The former depends on the induction density in the iron, the 
frequency of the alternating current, and on the volume and 
nature of the magnetic material in question. No amount of 
lamination will get over this loss. 

The latter source of power loss is dependent on the extent of 
lamination of the iron and on the transverse section of each indi¬ 
vidual portion of the core. 

Now by employing sufficient iron in a test specimen, tvell 
laminated^ the eddy current loss can be made small compared with 
the hysteresis loss, whence any measurement now made of the 
total core or iron loss will for all practical purposes represent the 
hysteresis loss simply. This is the principle upon which the 
present method is based, but if greater accuracy be desired the 
results so obtained can easily be checked by one of the methods 
given in the above-mentioned work. 

It will thus be at once evident from the foregoing remarks that 
the iron employed in all electrical engineering appliances, but more 
particularly in alternating current ones, should be tested for 
hysteresis loss prior to being used in the construction of such 
appliances. The present method is one of the simplest and most 
expeditious ways of measuring the hysteresis loss in different 
samples of iron which may be to hand, and it is accurate enough 
for most practical purposes. 

Probably the most important direction in which the preceding 
remarks find an application is in transformer, alternator, and 
alternating current motor work. As the magnetic circuits of such 
appliances are built up of stampings-out of thin soft sheet iron, 
this latter is the form in which samples to be tested usually come 
to hand. Assuming therefore that a few large sheets of the 
material to be tested, the thickness of which usually varies from 
0-35 m.m. to 0*5 m.m. for transformers and up to about 1 m.m. 
for alternators, etc., is at hand, the first thing to do is to prepare 
the material for testing by constructing a small transformer out 
of it thus—■ 


350 


ELECTRICAL ENGINEERING TESTING 


Preparation of Iron Samples for Test. 

Cut such a number of strips out of the sheet, each about 
12" X 2" as will make four equally thick piles, each containing 
the same number of strips, placed on the top of one another like 
the leaves of a book, to a thickness of, say, J" and each weighing 
about 2 lbs. 

Xow remove any burr from the edges of each by means of 
a file, and weigh all the strips, noting the total weight of iron JV 
which should preferably be 8 or 10 lbs. 

Next varnish one side of each strip with thin shellac varnish, 
and when dry assemble into four equal piles with varnished faces 
all pointing one way. Bind each pile, to within 1J" of each end, 
with a layer of thin prepared tape, when each will be ready to 
receive the magnetizing coils. It will be noticed that each strip 
is insulated from the next by the equivalent of one layer of thin 
varnish, which is all that is needed. Next make a thin rectangular 
cardboard tube about 4" to 4^" long for each pile and capable of 
just slipping easily over it. Wind each of these with two distinct 
coils of, say. No. 18 double cotton-covered copper wire, each coil 
consisting of two layers and the two coils wound one over the 
other. Place the four bundles of strips with their coils in position 
so as to form a rectangular frame of iron with adjacent ends 
interleaved, so to speak, and clamped together so as to form a 
compact joint of low resistance. Join the four coils of each set 
together so that they would help one another in magnetizing the 
ring and the specimen is then ready for test. Note the* total 
number of turns Np and Ns on both primary and secondary 
coils respectively, also the cross section S sq. c.ms. of iron in 
the frame, i. e. thickness of strip x by number side by side 
X width of strip, and the mean length of the path of a line of 
force right round. 

Apparatus. —Iron core on frame I to be tested and wound with 
the two distinct (closely-wound) primary and secondary coils 
P and S.^ Siemens electro-dynamometer or Parr direct reading 
dynamometer ammeter A (Fig. 577); non-inductive Wattmeter IF; 
non-inductive rheostat R (p. 597); switch K ; electrostatic volt¬ 
meter F; PohFs commutator!) (p. 584), or other suitable change- 


ELEGTRIGAL ENGINEERING TESTING 


351 


over switch for throwing V in quick succession across P or S, 
Source of alternating current supply preferably one the fre¬ 
quency of which is under control; a tachometer will be required 
in this latter case. 

Observations. —(1) Connect up as indicated in Fig. 128, and 
adjust the pointers of A, V and W to zero if they require it and 
levelling them where necessary. If the alternator is under 
control see that all the lubricating cups in use feed slowly and 
properly. 



Fig. 128. 


(2) Start the alternator up to its highest desirable speed, which 
is to be kept constant, then with R at its full close H and alter 
R and the excitation to give the smallest readable current on A. 
Note simultaneously the readings on A, W and V in quick suc¬ 
cession when across P and S by turning i) to P or /S' as the case 
may be, and the speed. 

(3) Repeat 2 at the same speed for eight or ten different 
currents A^ rising by about = increments to the highest desirable. 

(4) Adjust the current A to some convenient value, preferably 
one that will produce an induction of P = about 4000 lines per 
sq. cm. in I and keep this constant. 

(5) Now take a series of readings of W and F for about eight 
or ten different speeds, ranging from the greatest down to the 
smallest, noting the value at each. 

(6) Measure the resistances of the primary and secondary 
windings by means of a Wheatstone Bridge set, and tabulate all 
your results as follows— 

























352 


ELEGTEIGAL ENGINEERING TESTING 


Name . . . 


Date . . . 


Alternator: Periods per Revoln. K = . , » 


^nKN 

p = 2n-n =—per sec. 


Resistances (liot) Primary Rp = . . . ohms. 

„ ( „ ) Secondary R^ = . . . „ 

No. of Turns: Primary Np = . . . Secondary Ng 

Wattmeter Constant 


Section of iron Core 5 = . . . sq. eras. 
Weight of ,, „ ir = . . . lbs. 

= . . , Thickness of Strips = . . 


Speed of 
Alternator N 
revs, per min. 

Frequency n = 

per sec. 

60 

Current. 

E.M.P. 

Power. 

Approx, loss in 

Primary A^Rp. 

Mean Core Loss 

Total = 

W-A‘2Rp 

= H approx. 

Hysteresis loss 

per lb. of iron 

HIW. 

Mean Core 

Induction 

B- 

per sq. cm. 

Deflection 
on A. 

True Amps. 

S 6. 

Secondary 

Reading on 

W. 

True Watts 

(IF). 














(7) Plot the following curves— 

(a) Between H and E having E as ordinates and hysteresis 
loss H as abscissfe. 

(b) Between H and n having n as ordinates and hysteresis 
loss II as abscissae. 

(c) Between H and A having A as ordinates and hysteresis 
loss H as abscissae. 

Note. —E varies from 3000 to 5000 C.G.S. lines in ordinary 
transformers. With good iron HIW should not exceed J. 

Inferences. —State very clearly what you can infer from the 
results of your tests. 


(124) Separation and Measurement of Iron 
Losses in the Cores of Alternators, Trans¬ 
formers, Motors and other Electro-mag¬ 
netic Appliances. (Alternating Current 
Frequency Method.) 

Introduction. —The iron losses taking place in the cores of 
alternating current plant (e. g. alternators, transformers, motors, 
etc.) consist of those due to magnetic hysteresis and eddy or 
Foucault currents respectively. 

The Hysteresis Loss depends on the induction density in the 
iron core, the periodicity of the supply current, and on the 
volume and quality of the iron used, but in no way on the extent 































ELECTRICAL ENGINEERING TESTING 


353 


to which the lamination of the core is carried and increases 
with, but more rapidly than, the induction. 

Steinmetz gives the empirical equation for the work done on 
account of hysteresis as w — ergs per cycle of current and 
magnetization where rj equal the hysteretic constant which may 
vary from 0*001 to 0’003 for soft, annealed core plates, and 
^ = maximum value of induction density in lines per sq. cm. 

If (n) = periodicity of the supply or number of complete 
periods per second, the effective loss Wa due to hyteresis (per 
cub. cm. of core) will be 

Wa = r]nl3^'^ ergs per sec. = watts, 

on the assumption that the hysteresis loss per cycle is inde¬ 
pendent of the rate of cycle which the latest research shows to 



be not quite the case, though sufficiently so for practical pur¬ 
poses. Actually the hysteresis loss per cycle increases slightly 
with increase of periodicity, and from the above relation we 
see that for a given core, run at constant induction density 
Wb is oc n. 

The Eddy Current Loss depends on the strength of the 
induced eddy currents set up in the thickness of each lamina 
composing the core, and hence, by Ohm’s law, will vary as the 
square of such strength. The eddy currents are due to the 
varying flux through the core, and will depend on the rate of 
variation of this flux, i. e. on the periodicity (n). Thus the 
eddy current loss will vary in proportion to and the effective 
loss Wb due to eddy currents (per cub. cm. of core) will be 

Wb = watts, 

where K = a. constant taking into account the specific elec¬ 
trical resistance of the iron and the thickness of plate lamina. 

A A 








354 


ELECTRIGAL ENGINEERING TESTING 


Eddy currents tend to reduce the flux in a core due to their 
demagnetizing action and also cause a non uniform distribution 
over the sections of the laminae. l)ue to this, the hysteresis 
loss will be further increased with increasing values of (//), and 
will appear as an increase in the eddy current constant (A) in 
the present method over and above the calculated value. Espe¬ 
cially will this be the case if the insulation between core 
laminae is not all efiective. 

If = the total power in watts absorbed by any electro¬ 
magnetic appliances 

and JFo = the watts absorbed or expended in the exciting coil, 
then the nett iron losses 

= w,= w-iF„=frB+ 

watts. 



Now the coefficients rj and K can be found experimentally by 
testing the appliance at constant induction density ^ with 
alternating current at variable periodicity {n). This can be 
done by varying the speed of an alternator running at constant 
excitation^ for then the voltage V varies oc to the periodicity 
(n)j so that Vjn, and hence the flux, remains constant. On 


W 

plotting the values —- as a function of the induction B, the 

straight line ah is obtained corresponding to one particular 
value of ^ and of r/?i. From the curve ah and this value of 
B the coefficients r; and K can be calculated, and hence the 
hysteresis and eddy current losses respectively. 

Apparatus.—Electro-magnetic core I to be tested ; low-reading 
alternating current ammeter A ; wattmeter W-^ and voltmeter F, 
each independent of periodicity; frequency meter or, failing 
this, a tachometer for measuring the speed of the supply alter¬ 
nator E ; switch S and non-inductive variable resistance R, 
Observations. —(1) Connect up as shown in Fig. 130, levelling 
and adjusting to zero such instruments as require it. Start up 













ELECTRICAL ENGINEERING ' TESTING 


355 


the experimental alternator and see that the lubricating arrange¬ 
ments are working properly. 

(2) With a suitable ammeter, variable regulator, and switch 
in series with the alternator field across the D.C. supply, close 
the field switch TS, and adjust the speed to give the lowest 
readable value of periodicity [ii) on F (or smaller value by tacho¬ 
meter), and adjust the field regulator to give some suitable 
reading on V. Now note the readings of all the instruments, 
and particularly the value of F/n for future use. 

(3) Note the readings of all instruments for each of a series 
of periodicities (?i) up to the highest permissible, taking care 
that the value of Vjn (and therefore the value of the induction 

in I) is the same at each periodicity. 

(4) Repeat the above for the same range of periodicity, but 
for each of two other widely different values of F, giving corre¬ 
sponding values of the ratio F/?i (and hence inductions B) kept 
constant throughout each range of variation. 

Note. —If the appliance tested will safely stand, say, 150 
volts, then three values of the constant Vjn might be used, viz.— 


1 ^ 

50’ 


100 

50 


and 


or 3, 2 and 1 by suitable variation of 
50 ’ 


A 

field excitation, thus giving three corresponding values of B in 

IF 

the core. Further, since —- is not likely to exceed 1*5 or 2*0 

w 

watts per lb. in modern iron cores, a low-reading wattmeter 
will be required, unless the core is a heavy one. 

Tabulate as follows— 


Name . . . Date . . 

Supply A'teriiator : Periods per rev. P = . . . 

Core tested : Form or type . . , llateiial . . . 

Mean length of magnetic circuit I = cms. No. of magnetizing turns T = 

Net cross section of iron s = sq. cm-s. Res. of magnetizing turns r = ohms. 

Net weight of ir<. n v>= Ihs. Thickness of core laminations = 

Net volume of iron v = c.c. Width of core lamination = 


Alternator Speed 
in r.p.m. A”. 

Periodicity 

PN 

^ J3Q per sec. 

Volts. V, 

Calculated 
Constant vjn. 

Amps. A, 

Max. Ind. Dens. 

A 

B. 

Total Watts. 

W. 

CO 

CO 

O 

iJ . 

o 

O 

Sr 

c I 

3'L 

Ci. ^ Si 
^ o ^ 

c 

o 

u 

CH 









































356 


ELEGTEIGAL ENGINEERING TESTING 


(5) Plot curves having values of (n), as abscissae, with values 
of V, A and Wj respectively, as ordinates, for each constant Vjn 
taken. 

(6) Determine the hysteresis and eddy current coefficients, 
and from 0 draw a straight line, tangent to the curve relating 
Wi and n, separating the hysteresis and eddy losses, when 
ordinates of this line will represent losses due to hysteresis cc n, 
while the ordinate intercepts between it and the watt curve will 
represent losses due to eddy currents oc 

Inferences. —State clearly what you can infer from the above 
results. 


(125) Measurement of Magnetic Hysteresis 
by Ewing’s Hysteresis Tester. 

This instrument, a general view of which is seen in Fig. 131, 
and for a full description of which see Professor Ewing’s Paper 
in the Journal of the Institution of Electrical Engineers^ April 25, 
1895, has been designed to meet the want which has been felt 
of a means of testing the magnetic hysteresis of sheet-iron or 
steel in a simple and expeditious way suitable for workshop as 
well as laboratory use. A few strips of the sheet metal to be 
tested are cut or stamped, five-eighths of an inch wide and three 
inches long. They are filed to the exact length when clamped 
in a gauge, which is provided with the instrument, and are 
then inserted in a carrier which is made to revolve by turning 
a handle. The carrier turns between the poles of a permanent 
magnet, which is suspended on a knife-edge. In consequence 
of the hysteresis of the specimen the magnet is deflected, and 
the amount of its deflection is observed by means of a pointer 
and scale. From this deflection the hysteresis of the specimen 
is determined. The magnetic induction is practically the same 
in all specimens, notwithstanding differences in the permeability 
of the iron, on account of the comparatively large air-gap 
between the specimen and the magnet poles. 

Two standard samples are provided with the instrument. 


ELECTRICAL ENGINEERING TESTING 


357 


having stated amounts of hysteresis. The test of any other 
specimen is made simply by comparing the deflection produced 
by it with the deflections produced by the standard samples. 
This serves to determine the hysteresis of any specimen in 
absolute measure. 



Fig. 131. 

The operation of the instrument is entirely mechanical, and 
requires no knowledge of electrical testing. 
































































358 


ELEGTEIGAL ENGINEERING TESTING 


(126) Measurement of the Impedance, React¬ 
ance, and Self-Induction of Alternator 
Armatures, Motor Stators, Transformer 
and other windings (by Alternating 
Currents). 


Introduction. —The following is a simple and an approximate 
method of finding the self-induction L of an inductive circuit. It 
depends on the fundamental relation subsisting between current 
(^) and impressed E.M.F. (V) in such a circuit, namely—- 

V 


or, as it may otherwise be written, 

Impedance = =V/A, 

where the angular velocity of the current n being its 

frequency in per sec. 

Since by definition, the coefficient of self-induction L of 

N 

any coiled circuit is = — where N is the total magnetic flux 


threading the coil and produced by a current A, it follows that 
if the inductive circuit encloses, and is surrounded by, a non¬ 
magnetic medium, the value of L calculated will be the same 
for all values of A. If, however, it encloses an iron core, any 
variation of A will produce variations in the permeability of, 
and consequently the flux in, the core, and the value of L will 
varj with A. 

Thus the impedance and self-induction of the primary of a 
static transformer, and of the stator winding of an induction 
motor will decrease as the secondary load of the former and 
B.H.P. developed by the latter increases, owing in each case 
to alteration of current and core flux. The same efl[ect occurs 
with the armature of an alternator or of a synchronous motor, 
the impedance and self-induction of which will vary with— 

(1) The armature current, since the core flux and permeability 
will vary inversely together as the current changes. 

(2) The magnetization of the core due to any variation of the 
field-magnet strength. 

(3) The type of winding used, i.e. whether “distributed” or 





ELECTRICAL ENGINEERING TESTING 359 

“ concentrated,” the former having small self induction owing 
to the circuits being partly in both favourable and unfavourable 
positions for linking with the flux, the latter having a large 
self-induction, due to all the circuits in certain positions in the 
revolution linking up simultaneously with the flux. 

(4) The reciprocal of the length of air-gap between armature 
core and field poles. 

(5) The exact position of the armature relatively to the field 
poles, especially with windings concentrated into single slots. 

(6) The induced currents in the pole pieces and field windings 
due to the armature current. 

In view of the above considerations, it will therefore be 
obvious that the value of the impedance or self-induction of 
the armature of an alternator available for calculation can only 
be a mean value as obtained in the manner indicated in the 
present list. 

Two methods of procedure are possible, according as to the 
mode of obtaining the Ohmic resistance R. 

(a) R may be measured in the usual way on a Wheatstone 
Bridge either before or after the test, in which case a measure¬ 
ment of the current at a known voltage, or vice versd together 
with (w), at once gives the self-induction L. 

(h) R may be obtained by Ohm’s Law in terms of a continuous 
current and pressure when this latter is available, and therefore 
no Wheatstone Bridge is necessary. This method of procedure, 
which is the one adopted in the present instance, has the further 
advantage that in cases where R is liable to heat up, due to the 
current, its value will be obtained correctly, which would not be 
so if obtained by the bridge. 

The following precautions should, however,be carefully observed, 
and are practically the same as appear in the meisurement of the 
resistance of an electric glow lamp while running (vide p. 47). 

If the voltmeter is shunted across the terminals of Z, then its 
reading is correct, but the true current through L which is required 
=-■ ammeter reading - voltmeter current. If,- therefore, an electro¬ 
static voltmeter is employed this correction does not occur, but if 
a hot wire voltmeter is used, the correction should be made, as the 
voltmeter current is not usually negligibly small compared with 
the main current. 


360 ELECTRICAL ENGINEERING TESTING 

If the voltmeter is across the ammeter and L combined, then 
the true voltage across L = voltmeter reading — voltage absorbed 
in ammeter. Owing to the low resistance of the last-named 
usually, this correction is negligible, but must be made if the 
ammeter resistance is considerable. In this arrangement we also 
have— 

True self-induction of coil — calculated L — self-induction of 
ammeter. 

The test can be performed either using the same voltage 
from the direct and alternating sources and noting the relative 
currents, or employing the same current and observing the relative 
direct and alternating volts necessary to send this current through 
the circuit, the frequency in either case remaining the same. 

In the present instance the latter way will be adopted as being 
more readily applied. 

Apparatus. —Inductive circuit L to be tested ; alternating 
current voltmeter F, preferably electrostatic; Siemens electro¬ 
dynamometer or A.-C. ammeter A 
(Fig. 251); variable non-inductive 
resistance i? (p. 598); change-over 
switch S (p. 582); alternator P, 
with its tachometer; direct current 
dynamo or secondary battery D. 

Observations. —(1) Connect up 
as indicated and ad j iist the pointers 
of A and F to zero. Before starting 
see that all lubricators in use feed 
slowly. 

(2) Make R as large as possible 
and switch S over to P, adjusting 
A to the maximum current which 
L will carry. Note the currents amps, and the volts across L. 

(3) Turn S over to R and adjust R and the speed of P so as to 
again obtain the same current A amps. Note the volts (F,) and 
the speed of alternator. 

(4) Open S and make R as large as possible again. Repeat 
2 and 3 for about ten different decreasing values of current to 
the smallest convenient, and keep the speed of P constant through¬ 
out, its excitation being varied, if necessary. 


L 
























ELECTIilCAL ENGINEERING TESTING 


361 


Note. —If the double pole change-over switch S is not avail¬ 
able, the common circuit containing A, L and R can be closed 
to D through a single pole switch, and a series of pairs of values 
of A and V first taken in order to obtain the ohmic resistance 

of L. r can then be substituted for i), and a similar series 

taken with alternating currents of the same scale values on ^ 
as before between 0 and the maximum L will carry at constant 
frequency. 

(5) If the inductive circuit has a removable magnetic core, 
as, e g.y in the central movable core open-magnetic-circuit type 
of choking coil. Then operate obs. 2-4 above, or the alternating 
current series only of observations mentioned in the Note above, 
with the core (a) central in the coil, (6) removed altogether 
away from the coil. 

(6) If the inductive circuit L (Fig. 132) consists of the 
armature of an alternator the impedance and self-induction of 
which is required, the A.C. supply should have the same 
periodicity as the normal value for the machine under test. 
Then, with the field magnets of the machine under test, unexcited 
vary R so as to obtain about a quarter, half, three-quarters and 
full-load currents (A) through the armature, noting the corre¬ 
sponding readings of V at each of a series of positions of the 
armature throughout a fraction of a revolution equal to half 
the polar pitch. 

(7) Repeat (6) with normal field excitation and tabulate as 
indicated. 

(8) Plot curves for each fraction of full-load current having 
impedance and self-induction respectively as ordinates with 
positions of armature throughout the half-polar pitch as 
abscissae. 

Inferences.—State clearly all that can be deduced from the 
results of the test, and find the average value of the impedance 
and self'induction, and tabulate as follows-^ 



362 


ELECTRICAL ENGINEERING TESTING 


Name . . . Date , 

Alternator: Speed iV = , . , R.p.m. Periods per Revolution K = . , . 

KN 

Frequency n = = . . . per sjc. p = 2nn = . . . 

Nature and form of coil tested . . . 


Position of Core 
or Armature. 

Field Amps. 

For test of 
Armature. 

Current. 

Amps. 

A (true). 

Voltage 

Ohmic Resistance 

R = —ohms. 

1 A 

1 

Inductive 

Resistance 

Lp. 

Impedance 

y 

A 

II 

• 

p: 

c5 

•4^ 

Angle of Lag 

0°. 

Time Constant 1 

of Coil 1 

T = LjR. 

II 1 

^ 1 

-11 

Direct 

Vir 

Alternating 














(6) Plot curves having values of L and Va as ordinates and 
the corresponding currents A as abscissae in each series. 

Inferences. —What can you infer from your experimental 
results ? On what does* the self-induction of an alternating 
current circuit depend 1 Show how the formula given for L can 
be obtained. 


Self-induction by Rowland’s Alternating 

Current Method. 

General Remarks. —Every electrical conductor possesses three 
qualities, namely, (1) Electrical resistance^ which depends on the 
size and material of the conductor. 

(2) Electrical capacity, depending on its surface and form, and 
on the specific inductive capacity of the surrounding media 
(i.e. dielectric). 

(3) Electrical inductance, which depends on the shape and form 
of the conductor and on the magnetic permeability of the 
surrounding media. 

This last-named property may be of one or other of two 
kinds, namely, either the self-induction of the conductor on itself, 
or the mutual induction of the conductor and a neighbouring 
circuit on one another. 

The quality (1) above is usually easily obtained, except perhaps 
in the case of electrolytic liquids, and this only in one or two 
































BLTJGTRIGAL ENGINEERING TESTING 


363 


methods; the other qualities are much more difficult of deter¬ 
mination, and almost numberless methods have been devised for 
obtaining them. 

In general it may be remarked that relative or comparative 
measurements are more accurate than absolute ones, though the 
final results might be completely vitiated by comparing with 
an inaccurate standard. The former remark results in the 
difficulty experienced in accurately measuring an alternating 
current, and from the fact that its E.M.F. wave may differ 
considerably from that of a sine curve. 

In employing condensers in methods of measuring self and 
mutual induction, considerable difficulty is usually met with in 
the phenomena of electric absorption. Professor H. H. Rowland 
has found that this can be represented by a resistance placed in 
series with the condenser, which resistance is a function of the 
square of the current period. 

I 

1 

(127) Absolute Measurement of Self-induction 
(by Alternating Currents). 

Introduction.—The following method of measuring the self- 
induction of a coil, due to Professor Rowland, necessitates the 
employment of ordinary single phase alternating currents of 
electricity with an electro-dynamometer specially constructed, so 
as to be as sensitive as possible. It is possible to make such 
an instrument, having its fixed and moving coils connected up 
to two distinct pairs of terminals, that it will detect O’OOOl 
of an ampere with a self-induction in the suspended coil not 
exceeding 0'00075 henries, and in the fixed coil of not more 
than 0-0006 henries, capable of carrying about 0-1 ampere 
^ comfortably. 

Such an instrument obviates the necessity for using large 
currents in order to obtain accuracy and sensibility. If (d) = 
the deflection of the swing coil from zero when its plane was 
perpendicular to that of the fixed coil, (7j and C 2 — strengths 
of the alternating currents flowing through the movable and 
fixed coils and having an angle of phase difference 6. Then 
d oz cos. 6. 


364 


ELECTRICAL ENGINEERING TESTING 


The principle of the present method consists in adjusting C\ and 
C 2 to a phase difference of 90". 

In deflection methods cos. $ is greater than 0, while for zero 
methods cos. ^ = 0. In the former the self-induction is obtained 
in terms of resistance and the angular velocity of the current 
p = X frequency, which consequently require that (n) the 
frequency should be constant and accurately known to at least 
1%, in which case the results will probably agree to within about 
the same amount. 

Apparatus. —The electro-dynamometer, of which (jT) is the 
fixed, and (m) the moving coil; non-inductive resistance r ; 

source of alternating current; 
and the self-induction L to 
be measured; switch S', rheo¬ 
stat Rh (non-inductive). 

Observations. — (1) Con¬ 
nect up as indicated in Fig. 
133, placing L and a non- 
inductive resistance U in 
series with the moving coil 
(m), and the combination 
across the terminals of a re¬ 
sistance (r) in the main circuit in which the fixed coils are also 
placed. 

(2) With the moving coil adjusted to zero, Rh at a maximum, 
close S, and obtain a convenient deflection d by adjusting Rh and 
the non-inductive resistance R in the moving coil circuit. Note 
this deflection {d) and the speed or periodicity in) of the 
alternator and the added resistance R in circuit with L. 

(3) Remove the self-induction (A) which is being tested, and 
add a non-inductive resistance to the swing coil circuit such that 
the same deflection ( 0 ?) as before is reproduced. Note the 
new resistance in the circuit, the frequency (n) -^s^per sec. 
being the same as before. 

(4) Calculate the self-induction L tested from the relation 

L = (R + r) ^ J{R' -R) {R -h r) secohms, 

y 2Trn 

and tabulate as follows— 


















ELECTRICAL ENGINEERING TESTING 


365 


Name . . Date . . . 

Alternator : Periods per revolution K — . . . p = 2nn. 

Self-induction tested: Nature . . . 


Sjteed of Alternator 

N revs. per. min. 

Frequency 

K N 

n = per. sec. 

60 

Resistances. 

Deflection 

id). 

Self-induction. 

r. 

R. 

R'. 

L. 

Mean 

L. 







— J 


(5) Repeat 2—4 for different deflections {d) at constant fre¬ 
quency. Also for different frequencies with the same deflection. 

N.B.—Great care must be taken to keep the frequency constant 
throughout any pair of readings. 


{128) Comparison of Two Coefficients of Self- 
induction (by Alternating Currents). 

Introduction. —When an accurate standard known self-induc¬ 
tion is available the value of an unknown induction can be more 
accurately determined by comparison, for in this case the 
measurement is independent of frequency. The following method 
due to Professor Rowland is a zero one, and is similar in many 
respects to the preceding method, though this was a deflection 
method. The effects of induction and electrostatic action of the 
various parts of the circuit on one another must be carefully 
avoided as much as possible, and in this connection it should be 
remembered that a twisted twin lead possesses the latter quality. 

Apparatus. —The unknown self-inductions to be compared, 
with a standard L (known) ; non-inductive resistances r, 7i*, R! 
and rheostat Rh ; switch S ; 
source of alternating current 
E ; electro-dynamometer of 
which (/) is the fixed and 
i^n) the moving coil. 

Observations. — (1) Con¬ 
nect up as in Fig.134, the coils 
Zj Zg being together, and 
adjust the moving coil to 
zero. 

(2) With Rh large close and adjust the current to a 



f \ 


) 




-wvw- 




/?/) 


—O I O—AAAAA/" 


Fig. 134. 


































366 


ELECTRICAL ENGINEERING TESTING 


convenient value, and adjust R, R' and r so as to get no deflection 
of the moving coil for the currents in f and m. 

(3) Calculate the self-induction in terms of the standard from 

the relation ^ ^ and tabulate as follows— 

Zi r 


Name . . . Date . . . 

Alternator: Periods per Revolution K = . . . p = 2nn, 
Self-induction tested : Nature . . . 


Speed of 
Alternator 
(N) - 

revs, per min. 

Frequency 

KN 

” “ 60 
per sec. 

Knovra 

Self-ind. 

L. 

Resistances. 

Ratio 

L/Li. 

Unknown Self-ind. 

R. 

R'. 

r. 

lx 

secohms. 

Mean 

ii. 











(4) Repeat 2 and 3 for different values of L, R, R^ and r at 
constant frequency (n). Also for the former constant at different 
frequencies. 

Notes. —When equal self-inductions are being compared it is 
found that the accuracy depends only on the sensitiveness of D 
to changes in (A -I- i?'), and this instrument may be such that it 
detects differences or changes of 0'01%! 

If it is noticed that increase of frequency causes a diminution of 
the resulting value of then the electrostatic capacity of the 
turns of the coils on one another is asserting itself and cannot be 
avoided. Considerable care should be taken to avoid this source 
of vitiation as much as possible, and also that due to heating of 
the conductors, etc. To minimize the former error, use short 
small wire leads, some distance apart, and not twisted twin lead. 

> » 

(129) Self-Inductions in Series and Parallel. 
Experimental Determination of Laws 
of Combination. ^ 

Introduction. —An electrical circuit may contain any or all of 
the three qualities—self-induction, capacity, and ohmic resist¬ 
ance. The last-named is always present, and may be combined 
with one or both of the former. 

Let us suppose that no capacity is present, then the circuit,* 
whether consisting of several distinct portions either in series or 























ELECTEIGAL ENGINEERING TESTING 


367 


parallel, or a combination of these, each having its own particular 
self-induction and ohmic resistance, possesses on the whole one 
definite effective value of induction and resistance, which may be 
termed the “ comhmed, equivalent^ or effective^'' self-induction and 
ohmic resistance of that circuit composed of such detailed portions. 

In alternating current work it is of great importance to know 
the way in which various combinations of self-inductions and 
ohmic resistances will affect the working conditions of a circuit. 
The present test is devised with a view to elucidating these points 
for the three different forms of circuits or combinations, as 
follows— 

(a) The self-inductions and ohmic resistances in simple series 
only. 






I 

Fig. 135. 

{P) Self-inductions and resistances partly in parallel and in series. 

(y) „ ,, „ in parallels only. 

Apparatus.—Source of alternating current, preferably an inde¬ 
pendently driven alternator D, the exciting circuit (not shown) 
and the speed of which are under control; Siemens electro¬ 
dynamometer (Fig. 251) or Parr direct-reading alternating current 
ammeter A (p. 572); hot wire or electrostatic voltmeter F; non- 
inductive rheostat r (p. 598); switch speed indicator; four 
coils A—D to be experimented upon, as nearly alike as possible, 
and c.ipable of being used with or without iron cores, which 
latter are also similar in all respects. 

Observations.—(1) Connect up as in Fig. 1351., and adjust the 
pointers of all the instruments to zero, levelling such as require it. 

(2) Connect coil A (only) to W (I.) and remove its iron core. 





















368 


ELEGTRIGAL ENGINEERING TESTING 


See that all lubricating cups are feeding slowly, then S being 
open, start up to a convenient speed, N revolutions per minute, 
which must be kept constant throughout all the tests. 

(3) With (r) at its maximum, and the excitation low to start 

with as a precaution, close S, and adjust r and the excitation so 
as to give f, and full or maximum safe current through 

coil A successively at constant speed. Note the reading of the 
voltmeter V and ammeter A simultaneously at each, then open S. 

(4) Repeat 3 for each of the coils R, C and I) singly without 
their cores. 

(5) Repeat 3 for the coils connected 2, 3 and 4 in series 
respectively, i. e. as in a. 

(6) Repeat 3 for the coils connected 2 in series and 2 in 
parallel, i. e. as in jS. 

(7) Repeat 3 for the coils connected 2, 3 and 4 in parallel 
respectively, no iron cores being used in any of the cases above. 

(8) Repeat 2-7 with iron cores in all cases, if possible. 

(9) Repeat 2-8 with an entirely different speed, and therefore 
frequency of alternating current. 

(10) Measure the ohmic resistance of each of the coils A—Ehy 
a Wheatstone Bridge in the usual way, and calculate the self- 
induction (Lc) of the coil or combination of coils from the relation 


V 


amperes. 


where = combined or effective ohmic resistance of the coil or 
combination of coils obtained by employing the ordinary rules 
for the combined resistance of coils in series and parallel. Tabu¬ 
late your results as follows— 


Name . . . Date . . . 

Alternator; Speed N = . . r.p.m. Periods per Revoln. K = . . 

KN KN 

Frequency of the Current ^ = • • • per see. p = 2Trn = 27r gy = • • • 


Arrangement of Coils 
measured. 

Current. 

Volts V. 

Effective ohmic 
Resistance of Circuit 
tested A. 

o 

o 

Q • 

s ^ 

o ^ 

a 

II 

c 

c3 

0 * 

to 

3 

<*-i 

O 

to 

Impedence. 

Self-induction. 

Dynamometer 

reading. 

CC 

1 

I 11 + 

S t: 

^3. a, 

0) s. ^ 

> 

Calculated. 

9 11 

<D H 

IN 

1 

S' 

> 

Calculated. 


















































ELECTRICAL ENGINEERING TESTING 369 
L For pure series combinations such as (a) Fig. 135, show that 


V 




or generally that J^^Lf and therefore that the 

total effective self-induction Zc = sum of the individual self- 
inductions composing the circuit. 

II. For series-parallel combinations such as {jl) Fig. 135, show 
that 

• 2 = jLjfTuy 


s/(2Z)2^2 + (2/e)2 

III. For pure parallel combinations such as (y) Fig. 135, show 
that 


V 

A 






where I is the impedence of each parallel branch. 

N.B.—In the present test it is assumed that the coils are 
incapable of having any mutually inductive action on one another, 
and consequently they must be arranged not to have such when 
making the test. 


The Electrostatic Capacity of Electrical 

Wires and Cables. 

General Remarks. —The condition for obtaining an electro¬ 
static capacity is the passage of a quantity of electricity into one 
of two conducting bodies which are separated by an insulator. 
Such an arrangement constitutes what is commonly termed an 
electrical condenser, the two conducting bodies being called the 
“ coatings,'' and the separating insulator the “ dielectric " of the 
condenser. Now it will be obvious that any insulated electrical 
wire or cable in contact with earth or its equivalent will form a 
condenser, the inner conductor or wire and earth being the two 
coatings, and the insulation of the cable the dielectric. In the 
case of an insulated cable possessing only one core, whether 














370 


ELEGTBIGAL ENGINEERING TESTING 


consisting of one wire or a strand of wires, we shall obtain one 
particular definite capacity with a definite position of the cable. 
In other words, the capacity will depend on the geometrical form 
of the wire, so that if it was coiled up in a tank of water the 
capacity would not be the same as if it was laid out straight on 
the ground. 

The actual value will depend in addition on the length and size 
of cable, and on the thickness of the insulation and its specific 
inductive capacity. The latest forms taken by cables, in which 
one conductor completely envelops another, but is insulated from 
it by a fairly uniform stratum of insulating material between the 
two, possess this property of having an electrostatic capacity in 
a more marked degree than the simple form mentioned above. 
Such a concentric cable, as it is termed, possesses a definite 
capacity per unit of length, and which is independent of how the 
cable is placed, i. e. whether coiled or straight. For continuous 
currents the capacity of cables or wires is of no practical import¬ 
ance, but for intermittent or alternating currents the case is 
otherwise. In submarine telegraphy the cable has naturally a 
very considerable capacity, while the current is intermittent; 
consequently when the circuit is closed so as to send a message, 
the cable has first to be charged by the sending battery before 
any current arrives at the receiving end for actuating the receiv¬ 
ing appliances. This may take some seconds, depending on the 
length of cable, i. e. on its capacity. Thus the effect of capacity 
in such an instance is a detrimental one, giving rise to what is 
called inductive retardation” and diminishing the speed of 
signalling. Here, in the above instance, we have the case of a 
single cable stranded conductor of which the copper core forms 
one coating, the iron sheathing and water the other. 

With concentric cables, it has already been remarked that their 
capacity is greater than with single cables for equal lengths and 
section of conductors in the two cases; but the former possess the 
advantage that whereas the “outward” and “return” leads are very 
close together, in fact one encircling the other, their inductive 
action on telegraph and telephone wires in the vicinity is practically 
nil, as the external magnetic field produced is very small. This 
is of great value in alternating current distribution, for since the 
magnetic field produced by such currents alternates rapidly in 


ELECTRICAL ENGINEERING TESTING 


371 


direction with the alternations of current, the inductive action of 
alternate current cables on such wires would otherwise be great. 
The capacity of a concentric cable can at once be calculated from 
the analogy to a cylindrical condenser as follows— 

Assuming the two conductors to be both concentric and cylind¬ 
rical, let A = radius of the inner surface of the outer conductor 
and r = radius of the outer surface of the inner one, and also let 
L = length of cable in centimetres. Then its capacity in farads— 

^ _ 2-413 KL 

^ 10^^ ^-logj^r 

where /ir= specific inductive capacity of the dielectric or insulat- 
ing material, which for paper =1*86 about, for india-rubber 
(pure) 2-34, vulcanized 2-94, for gutta-percha 4*2, and resin 2*55 
about. 


R 

It may be noticed that since (logj^T? — logjo?*) = logj^ — the 

units in which the radii are measured is quite immaterial, so long 
as the same is employed for each; the diameters D and d corre¬ 
sponding to li and r, may be used instead if we like, whence we 
shall have the 

KL „ , 2-413 LK 


n 2-413 

Capacity = _ x 


lO's log,//. 


Farads x 

10' 


logio7<< 


Microfarads 


reducing this to Mfds. per mile (statute) which = 160,933 cms. 


Capacity = 


2-413 x 160,933 K 


107 


X 


K 


log,//^ 25-75 log"/J per mile. 


Mfds. 


The capacity can readily be measured by means of the “method 
of mixtures ’’ due to Lord Kelvin, and which is one of the best 
for the purpose. A complete digest of this and other kindred 
methods will be found in Practical Electrical Testing^ p. 182, 
by the author, and they will not therefore be repeated here. 

It may, however, bo remarked that in testing the capacity of 
electric light and other cables by this method of mixtures the 
E.M.F. employed may conveniently be about 100 volts, and 
referring to Fig. 82, p. 184, of the above-mentioned work, the 
resistance ADR might be 100,000 ohms, and B connected to earth 
or tank if a single conductor cable is being tested. If it is a 
concentric cable this will not be immersed, and its two con¬ 
ductors at one end must be carefully insulated, while their other 
ends will form the two terminals of the capacity to be tested. 











372 


ELECTRICAL ENGINEERING TESTING 


(130) Measurement of the Electrostatic 
Capacity of Concentric—or Ordinary— 
Cables and Condensers. (Alternating 
Current Method.) 

Introduction. —When an alternating-current E.M.F. is placed 
across a condenser or, say, a concentric cable, a certain measurable 
alternating current flows into tlie condenser or cable, even though 
in the latter the two conductors are quite free of all connections 
to lamps or any other appliance throughout their entire length. 
Moreover this current, which is called the “ capacity current of 
the cable, is not in phase or step with the periodic impressed 
E.M.F., but leads in advance of it, and constitutes what is called 
a Wattless or idle current, to distinguish it from the load or usefid 
current which would flow in the cable when lamps or other 
appliances were switched on. In other words, it represents waste 
energy in the copper of the mains so far as the utility of the 
current is concerned, and the effect is always present with 
alternating currents. Thus it becomes of importance to know 
this idle or capacity current in order that its flow in the cable 
may not be mistaken for leakage current when no apparatus is 
connected to the cable. 

It should also be noted that this current is out of step or phase 
with the main current. 

The value of the capacity current in any cable can be deduced 
when certain constants are known. Thus— 

Let A = virtual or Vmean square value of the capacity current 
in amperes, 

y = virtual or Vmean square value of the E.M.F. impressed 
in volts, 

between the two conductors. 

C = capacity of the cable in farads, 
and p = 27m where n = frequency of the alternating current in 
periods per second, 

V 

then A — -Y = C 2 jV amperes, 




ELECTRICAL ENGINEERING TESTING 


B73 


being the effective resistance to the passage of the current or 

reactance of the cable or condenser. 

If G is in microfarads per mile and L — length of cable in miles, 


then A = 


GLVp ^irnGLY 


amperes, 



106 - 106 

whence C = — mfds. per mile. 

The present test is a very practical one and can nearly always 
be applied if the working pressure V is available. 

Apparatus. —Cable or condenser to be tested (C); either a 
^Siemens electro-dynamometer, hot wire or Parr ammeter A (p. 577}, 
each of which will correct¬ 
ly measure the \/(uiean)^ 
value of the current; an 
electrostatic or hot wire 
voltmeter (F), preferably 
the former; tachometer 
for measuring the speed 
of the alternator D, and 

from it deducing the value of (n); switch S. 

Observations. —(1) Connect up as shown in Fig. 136, and adjust 
the pointers of F and A to zero, carefully levelling them if 
necessary. 

(2) Carefully ’"’■free ” and insulate the far end of the cable, and 
prepare the near end so as to make contact with the two 
conductors I and II. 

Note. —If a condenser is being tested I and II will now be its 
terminals. 

(3) Close S, and with D running at constant speed take some 
six or eight widely different values of F by altering the excitation 
of D, and note the corresponding values of A simultaneously 
with F. 

(4) Next run D at six or eight different speeds, keeping F 
constant by altering the excitation, and note the corresponding 
reading on ^ for each speed. 

(5) Calculate the capacity tested from the relations— 

106^ 


C = 


V’27rn 


mfds. for a condenser, 




























374 


ELEGTETCAL ENGINEERING TESTING 


C = 


L V'lTrn 


mfds. per mile for a cable, 


and tabulate as follows— 


Name . . . 


Date . . . 

Cable tested : Type . . . 

Maker . . . 

Size . . . Insulation 

Condenser: Typo . . . 

Maker . . . 

Dielectric . . . 


Length 

of 

Cable 

G) 

miles. 

Speed 

of 

Alter¬ 

nator. 

Frequency 

(n) 

sec. 

Volts V. 

j 

Inductive 

Reactance 

1 

Cp 

Current. 

Capacity of 

Reading 

on 

Instrument. 

Amps. 

A. 

Condenser 

in Mfds. 

Cable Mfds. 

per min. 











(131) Measurement of the Electrostatic 
Capacity of Short Lengths of Submarine 
and Electric Light Cables. (Kelvin Dead 
Beat Multicellular Voltmeter Method.) 

Introduction. —The effects of electrostatic capacity in a cable 
on the intermittent or alternating currents flowing in it have 
already been mentioned {vide p. 370), consequently it is desirable 
to obtain the value of its capacity. The following is a convenient 
and accurate method of measuring the capacity of any insulated 
conductor comprising a short length of submarine, telephone, 
telegraph, or electric light cable, and its great advantage lies in 
the fact that it is applicable to short lengths. 

The method, which is very analogous to the “ Siemens sub¬ 
traction method,” except that only one deduction is made, consists 
in charging a standard known condenser to a measured potential 
and observing the fall of this on connecting the cable as a 
condenser in parallel with it. The condition for maximum 
accuracy, i. e. when a slight error in reading the diminished 
value of potential has least effect on the final result, has been 
shown to be when the standard capacity is equal to that of the 
unknown, or when the diminished potential = half the original 
value. 

For accurate work it is necessary to employ two or three 
small corrections—one arising from the multicellular voltmeter 





















ELECTRICAL ENGINEERING TESTING 


875 


possessing a small capacity itself which varies with the deflection 
of the suspended needle vanes, being less for smaller deflections. 
It is of the order of about 10“® mfd., and in most cases can be 
neglected in comparison with the capacity of the standard 
and cable to be tested, at least when these are of the order of 
0*01 mfd. or greater. When, however, extreme accuracy is 
required, the potential-capacity curve of the voltmeter, which is 
supplied by the makers, must be referred to, and the capacities 
of it, for the deflections obtained, taken into account. 

Another correction is for loss of charge due to leakage occurring 
in the voltmeter, condenser, and cable; since during the time 
taken for the needle of the multicellular to come to rest after 
cutting off the battery or^charging E.M.F. and putting the cable 
in parallel, the potential may have fallen owing to leakage. 

It may therefore be necessary to determine the leakage of 
the voltmeter, cable, and condenser, which can be done as 
follows— 

(a) Charge the voltmeter to some conveniently large potential, 
and take readings of potential and time after disconnecting the 
charging source. Then the curve plotted with potential as 
ordinates and time as abscissae shows the rate of fall of potential 
at any time after the disconnection of the charging source. 

(h) Join the voltmeter and standard condenser in parallel, and 
repeat the preceding operations. Then the leakage from the 
condenser will be given at any time by the difference of the 
ordinates of the curves in a and h. 

(c) Join the voltmeter and cable in parallel and again repeat. 
From these results leakage of the cable can be found at any time 
by subtracting the ordinates of the curves in a and c. 

Preparation of Cable Ends. —The free ends of the cable 
should be bared of the outer insulation down to the pure rubber 
for a space of some 2J", and the rubber itself pared or tapered 
with a clean sharp knife for a length of about IJ" from the 
end; the ends should be carefully dried over a spirit lamp. 
One end should then be repeatedly painted with melted paraffin- 
wax (for some from the end) heated to a temperature not 
exceeding 100“ C. by means of boiling water. The other end of 
the cable after having a short well-insulated gutta-percha wire 
soldered to the copper core should be treated in a similar manner. 


376 


ELECTRICAL ENGINEERING TESTING 


This method of preparation if carried out carefully will prevent 
all end leakage. 

Apparatus. —Kelvin dead beat multicellular electrostatic volt¬ 
meter V (Fig. 240); Kelvin standard air leyden or condenser C 

(p. 616); cable L to be tested 
immersed in a metal-lined 
water-tank T ’highly insu¬ 
lated two-way key K (p. 586); 
battery B giving an E.M.F. 
of something like 100 volts; 
and a fine fuse F to act as a 
safeguard in case of accidental 
short circuit in the condenser 
or voltmeter. 

Observations. — (1) Care¬ 
fully prepare the ends of the 
cable as indicated abov’o and 
immerse the cable in the tank 
T, taking care not to allow the 
prepared ends D and d to got wet; these must be trained up 
out of the water. 

(2) Connect up as in Fig. 137, taking care that the insulated 
terminal I.T. of both V and C are joined as shown. Adjust the 
pointer of V to zero if necessary and see that a fine fuse 
is in F. 

(3) Press K to 2 to charge V and 0, and note the reading Fj 
on the voltmeter when steady and then release K. Now observe 
the reading for one or two minutes to see if there is any sensible 
loss due to imperfect insulation in V or C, and if so, whether it is 
small enough to neglect. 

(4) Close K to 1, and in fifteen to twenty seconds, which is 
usually long enough, note the steady diminished reading Fg on 
the voltmeter. 

(5) Repeat 3 and 4 with different charging E.M.F.s, and 
calculate the capacity of the cable L from the relation— 

c. - g^ g i - r-)* y. Ji-. - I'. 

where Ca — standard capacity, in this case 0-0025 mfd., and 
the capacities of the voltmeter at potentials Fj and Fg. 











































ELECTRICAL ENGINEERING TESTING 


377 


Name . . . Date . . . 

Standard Capacity : Type . . . Capacity = . . . mfds. 

Cable tested : Type . . . Length (?). • • Insulation . . . 


E.M F. 
used to 
Charge. 

Voltages. 

Capacity of Voltmeter. 

Capacity of Cable. 

Initial Vi. 

Final V^- 

at Fi. 

1^2 

at V 2 . 

Cj^ Mfds. 

Or 

-I Mfd. 

1 

per mile. 









(132) Measurement of the Electrostatic Ca¬ 
pacity of a Concentric Cable Ballistically. 
(Standard Magneto Inductor Method.) 


Introduction. —When some standard form of magneto inductor 
is available, the form devised by Dr. W. Hibbert being a very 
convenient and easily manipulated one, the capacity of a concen¬ 
tric or other electric light cable can be readily determined, pro¬ 
viding a few other additional pieces of apparatus are available. 
The reader should note the general introductory remarks on p. 
369 concerning the capacity of cables in general, and also those of 
the alternating current method of measuring the capacity of cables. 

The present test can be employed for finding the capacity of 
cables in tanks and of ordinary and concentric mains. As, how¬ 
ever, the former are best 
tested by the “ method of ^ 

mixtures” (p. 371), we shall 
here only consider the test 
of a concentric cable by this 
inductor method. 

Apparatus. —• Standard 
inductor to bo tested I 
(Fig. 138); sensitive ballis¬ 
tic galvanometer G ; con¬ 
centric cable to be tested 
C, of which A".is the free 
and well-insulated end, E 
the inner conductor, and 0 the other; box of known resist¬ 
ances r; battery R, of known E.M.F., or, if this is unknown, 
a standard voltmeter to measure the P.D. ; two-way spring 



/- ~i 




B 




Fig. 138. 











































878 


ELEGTBIGAL ENGINEERING TESTING 


tapping-key K (p. 586) ; ordinary spring tapping-key ; damp¬ 
ing-coil with its cell and key. 

Observations. —(1) Connect np as in Fig. 138, and adjust the 
galvanometer needle to zero, carefully prepare the free (far) end 
of the cable, viz. F, in the manner described on p. 375, and also 
the (near) end as well; by so doing the vitiation of the results 
from leakage across the cable ends will be avoided. 

(2) K\ and 71^2 being open, adjust r to a low value, such that 
pressing nearly a full-scale throw is obtained on slipping 
down 7. Note this value of and the box resistance r ohms. 

(3) being open, adjust the voltage of the battery B to such 
a value that on closing 772 for two or three seconds, then open¬ 
ing it, and immediately closing 771, a first throw 7^ is obtained 
on discharging G, as nearly as possible equal to the former. 

N.B.—Two or three throws should be taken iu both 2 and 3, 
and the means noted as being more accurate. 

(4) Obtain the mean throw on the charge in a similar way by 
first closing 771 for a few seconds so as to completely discharge 
Gy and then opening it and closing 772 afterwards. 

Note. —Care must be taken that G is each time discharged 
before taking the charge throw. 

(5) If possible employ three or four different voltages and 
repeat 2 and 3 with each of them, keeping the deflections 7j 
and dc about equal to one another, preferably by varying r to suit. 

(6) Calculate the capacity of the cable tested from the relation 

rfr “‘-rofarads, 

where F = total magnetic flux in the air-gap of the inductor and 
R = total resistance in ohms of the inductor circuit. 

Tabulate as follows— 


Name . . . Date . . . 

Standard inductor . . . ; turns N = resistance rj; = . . . ohms. 

Galvanometer resistance = . . . ohms; Total Flux F = . . . C.G.S. lines. 
Cable tested: Type . . . Maker . . . 

Length of Cable L = . . . miles. Section (for reference only) = . . . sq. ins. 


Mean first 
throws. 

Resistance in Ohms. 

P.D. if 
variable 

V. 

Capacity in 
Microfarads 
0. 

Mean 

Capacity 

C. 

Capacity of 
Cable in 
Mfds. 
per mile 

qi. 

c?i. 

dQ. 

In box 

r. 

Total 

R = r + r^ + G. 




























ELECTRICAL ENGINEERING TESTING 


379 


Inferences. —Show how the relation given in 6 can be ob¬ 
tained, and state any assumptions made in obtaining it. Is any 
correction required for greater accuracy in the relation for C ? 

(133) Measurement of the Electrical Power 
absorbed in Alternating Current Inductive 
Circuits. (Three-voltmeter Method.) 

Introduction. —The measurement of alternating current power 
depends on the nature of the external circuit. Thus, if this 
circuit is non-inductive^ then the true power W = amps. 
(A) X volts (F), the former flowing in the external circuit at 
a terminal potential difference (F). 

If the circuit is inductive, and every circuit is to some slight 
degree, then IF = -I F cos where ^ = angle of phase difference 
of the current A behind the voltage F and the product X F) 
is called the apparent power absorbed. 

The measurement, therefore, of this electrical power accurately 
is more difficult than that in the case of a direct-current circuit, 
owing to the effects of self and mutual induction and capacity 
which appear in alternate-current (A.-C.) working. In such a 
case a Wattmeter may be used, but it must be practically non- 
inductive to give accurate results. Another method to employ, 
which will give accurate results even though most of the circuit 
is highly inductive, is that known as the “ three-voltmeter 
method^^’ and it has the advantage that only one A.-C. voltmeter 
is required, though three similar ones may be used if available. 
By it the true power absorbed by the circuit may be obtained 
with any degree of accuracy desired by using an accurately 
graduated voltmeter, and by carefully repeating the readings 
two or three times and noting the mean in each case. 

The three-voltmeter method, which was simultaneously sug 
gested by Prof. Ayrton, Dr. Sumpner and Mr. Swinburne, gives 
a true measure of the power given by any current, whether har¬ 
monic or otherwise, to any circuit, inductive or not. It has the 
disadvantage that, as the differences of squares of quantities is 
being taken, a small error in the quantities themselves may make 
a considerable error in the final result, especially if the angle of 
lag ^ is large. 


380 


ELEGTRICAL ENGINEERING TESTING 


The voltmeter used must be such as will not alter the voltage 
across the points to which it is applied. In other words, it , 
must have a high resistance compared with that between these 
points. 

An electrostatic voltmeter most accurately fulfils this condition, 
but if a hot-wire voltmeter is used (of relatively low resistance), 
the main current must be large compared with its own current, 
or an error will thus be introduced. 

With this apparatus we are in a position to investigate the 
following important characteristics of an inductive circuit formed 


Q 



by, say, a choking coil or the primary of a static transformer, 
etc., namely— 

(1) The true power absorbed in the whole and each part of the 
circuit. 

(2) The angle of phase difference between the current and 
both the supply and choker voltages. 

(3) The impedance, ohmic, and inductive resistances, and self- 
induction of the choker. 

The vector diagram for the circuit PR is that shown in Fig. 139, 
and is constructed as follows: set off a vector oa equal to the 
total voltage V across PR to any convenient scale. With radii 
oh = Fg and ha — and centres o and a respectively, draw 
arcs intersecting at join & to o and a and produce oh to meet 
a perpendicular from a in the point c. Then oha is the triangle 
of E.M.F.s for PR, and hca that for PQ where r-^ and L are 
the ohmic resistance and self-induction of the inductive portion 
PQ. Since QR is non-inductive, the current A and voltage Fg 
are always in phase, and hence by Ohm’s law Fg = Ar where 
(r) is the ohmic resistance of QR, and oe will be coincident with 
the current vector. Thus B will be the angle of phase difference 





ELECTRICAL ENGINEERING TESTING 


381 


between the current [A] and total voltage F, while 0-^ will be 
that between A and the voltage Fj. 

Note.— Errors in F, F^ or F 2 , or in the graduation of the volt¬ 
meter scale, will have least effect on the result when V-^ = F 2 , 
which is the condition for maximum accuracy, and the resistance 
r of QR should first be adjusted if possible to obtain this 
condition. 

Should the non-inductive resistance QR not be accurately 
known, or be likely to alter in value through heating due to 
the passage of the current A, then its equivalent in terms of F 2 
and A can be substituted in the formula. Hence, if A is the 
^mean square current in amps as given by a Siemens dynamo¬ 
meter or other direct reading alternating current ammeter, we 
shall have 


w = r/} Watts, 

for the true mean power given to the whole circuit PR, QR may 
consist of a bank of electric glow-lamps, as the resistance r of 
QR can vary if it likes with the different mean currents. 

It can easily be shown, in like manner, that the true mean 
power given to the inductive portion PQ of the circuit is 

The method is not based on any assumptions as to the nature of 
the current (whether periodic or otherwise) or of the circuit, which 
may contain either self or mutual induction, and capacity, or all 
three. It is based solely on the difference in phase between the 
current and voltage. 

If ^ = angle of phase difference or lag of the current behind 
the voltage, then if both are sine functions 


F2 - Fi 


2 _ 


]7 2 




1/2 _ 17 2 
COS. e = - 


V 2 
2 


Apparatus.— Alternator D and its exciting circuit; inductive 
portion PQ of the circuit in series with a strictly non-inductive por¬ 
tion QR ] two 2-way keys and K^) (p. 587); an A.-C. 
voltmeter (F); main switch (aS') ; A.-C. ammeter [A). For 
comparison of methods, A may be used, and also a non-inductive 
Wattmeter W for measuring directly the power used up in PR. 
Frequency meter /’connected across the supply i>. 





382 


ELEGTRIGAL ENGINEERING TESTING 


In the electrical circuit of the motor M may be used a volt, 
meter V^; ammeter cij^; switch rheostat R_E} source of 
continuous current E. 

Experiments. —(1) Connect up as shown in Fig. 140. Adjust 
the pointers of all the instruments to zero, levelling such as 
need it. 

(2) See that all lubricators in use feed properly, then start D, 
.running slowly. 

(3) Adjust its speed so as to get ^ of the max. — per sec., at the 
same time varying the excitation of D to alter its voltage ( V), so 



Fig. 140. 


as to send a convenient current (d) through PQ, Note A, and in 
quick succession (the speed being constant) the voltages F, and 
F 2 , across PR^ PQ^ and QB. (See Note above.) 

(4) Repeat 3 for about 5 frequencies between the max. and 
min. values possible, using the same current A in each case by 
suitably altering the excitation. 

(5) Repeat 3 at a constant frequency of about normal for five 
different current values, rising by equal increments up to the 
maximum allowed, by varying the. excitation. 

Tabulate your results as follows— 






















ELECTRICAL ENGINEERING TESTING 


383 


Form of Inductive Circuit PQ tested . . . 
„ Non-Ind. resistance QR used . . . 


1 Frequency (/). 

1 Amps. A. 

Voltages. 

Power in Watts absorbed 
in 

Power 

Factor. 

From 

Math. Tables 

Ohmic 

Ees. 

ft. 

■6 1=' 

'0 - 

<v ^ 
0 

Q 

X 

0 

11 

3p> 

ft. 

'•4-H 

0 

V 

V 

s 

0? 

4^ 

0 

c3 

0) 

3h 

93* 

rH 

o> - 
ft. + 

0 ^ 

® 'g' 
cj 1 = 

a 

33 \ 
> 

^ 11 

k’-' 

kX 

V. 

Vl- 

V2- 

PR. 

PQ. 

QR 

PR. 

By Ca’cu- 
lation. 

PR. 

PQ. 

Angle of 
Lag in 

rH 

rt 

4-> 

PQ. 

QR. 

PR 

PQ. 

Wattmeter W. 

93 

1 

1 

e« 

II 

S 

II 

<M 

S 

+ 

S 

li 

s 

m 

0 

0 

tx 

II 

8 

5 

+ 

M 

1 

s. 

^ bx 
l<M 

II 

§ 

t9 * 

+ 

93 ^ 

1 

II 

t/i 

0 

Q 

C 

(M 

99 

^ 1 
cr. «_ 
0 

1 

tx 

u 

e°. 

0i°. 

X 

11 

II 













1 









(6) Plot curves having values of A as abscissae with values of 
y 

cos ^1, L, and Wj, respectively, for the inductive portion 


PQ as ordinates. 

(7) Draw the vector diagram (Fig. 139) for the maximum 
current used. 

(8) Compare V with the algebraical sum (1^1+ Fg)* ^ 
with w. 

Inferences. —Prove the formula in column 7, and state any 
assumptions made in deducing it. What can be inferred from 
the results of the test and from the curves! 


(134) Measurement of the Electrical Power 
absorbed in Alternating Current Inductive 
Circuits. (Three-Ammeter Method.) 

Introduction. —This method, though inferior to that of the 
Wattmeter, is nevertheless instructive, and therefore a brief resume 
of it will be given here. As will be seen, it is very similar to the 
3-voltmeter method of measuring power, the formulae in the two 
cases being strikingly similar. There is, however, one chief 
difference between the methods, namely, that practically three 
ammeters are necessary for a satisfactory test, as large errors 





















































384 


ELECTRICAL ENGINEERING TESTING 


may occur if only one ammeter is employed and interchanged 
between the circuits, while in the case of the allied method one 
voltmeter can easily be made to do and no appreciable eiror 
need be introduced. The actual arrangement is shown in Fig. 
141, in which represents the circuit in which it is desired to 

measure the power taken up, A, and A 2 fire three non-inductive 
ammeters, at'deast A^ should be of this nature, while (r) is a non- 
inductive resistance connected as shown, and which is large 

compared with that of A^. 
Greatest accuracy will be ob¬ 
tained when A-^ = A 2 , and 
under these conditions it will 
be seen that (r) consumes 
about as much power as Q. 
Hence twice as much power 
has to be available at the 
source for operating this 
method as is taken up in 
PQ, but practically no excess voltage is needed in this case as 
it was in the 3-voltmeter method. If W 2 — the power in Watts 
absorbed by PQ, then 



and 




cos. 


= - A^^ - A2^} 


A 2 


2^ id 2 


where 0i = angle of lag of the current dg in the inductive 
circuit PQ behind the terminal voltage. 

It will thus be seen that the method is based on the difference' 
of phase of the various currents, and, as in the 3-voltmeter 
method, a small error in observing the currents introduces large 
errors in the answer. The possibility of such occurring can be 
minimized by using accurately calibrated non-inductive ammeters 
and taking the mean of three or four similar readings at each 
value of, say, d. If the value of non-inductive resistance (r) is 
not accurately known, or if it is liable to alter through heating due 


to the passage of the current, then its equivalent value — may 

^1 

be used instead in the formula, which will therefore become— 


(r= ^ ^.42 - - A/j Watts, 
















ELEGTEICAL ENGINEEEING TESTING 


385 


■where F=the voltage across the extremities of FQ. There is 
no objection to using a bank of incandescent lamps for (r), since 
the resistance may vary if it likes with the different mean 
current strengths. It will be observed that if the resistance of 
is appreciable, an amount of power may be absorbed in it 
which is comparable with that in FQ. In such cases the former 
must be deducted from the result as given by the above relation 
in order to obtain the true power absorbed in FQ alone. 

If (r) is accurately known we may dispense with A^ and put 
(r) directly across the mains, then on placing a voltmeter (prefer¬ 
ably an electrostatic one) across the mains as in the last instance, 
we may substitute the value of .dj in the first formula, when we 
shall have r 1 

Tr=irp-(-) 

The preceding remarks will be understood more clearly from 
the vector diagram, Fig. 142, for the circuit of Fig. 141, constructed 



as follows: set off a vector oa equal to the total current A in 
the main line to any convenient scale with radii o6 = J^and 
— j and centres o and a respectively, draw arcs intersecting 

A 

at b. Join 6 to o and a and produce ob to meet a perpendicular 
from a in the point c. Then oba is the vector triangle of 
currents for the main and both branches. altogether, while bca 
is that for the inductive branch only. Since r is non-inductive, 
its current A^ and voltage are always in phase, and hence by 

Ohm’s law A^ = If F equals the ohmic resistance of FQ 

and L the self-induction, then the energy or magnetizing 
component of the current A^ in FQ which is in phase 

with the voltage od across it, is be = while the idle 

or wattless component of the current quadrature with the 






386 


ELECTRICAL ENGINEERING TESTING 


voltage V is ca. The angle of phase difference between the 
main current A and V will be 0, and that between A 2 and 
the same voltage V across RQ will be 
From the geometry of Fig. 142 we see that 
A^^ = ^2 _|_ ^^2 _ 2 AAi cos Of 
Y 

but = — by Ohm’s law, and 

A^^=A^-{-A^^-2AjCos9, 

and A V cos 0 = total power given to the whole parallel circuit, 
the total power absorbed in the whole circuit is 

w= VX oc= ^2 __ watts, 

the power absorbed in the non-inductive branch 
Wi — A— A watts, 
and the power absorbed in the inductive branch 

W2 — VX he — Ua^ — Ay^ — A 2 ^) watts. 


where the power factor for the whole circuit = 


cos $ — 


A2 + Ay^ - A^^ __ r(^2^^^2_^^2) 


2AA. 


2AV 


and the power factor for the inductive circuit RQ = 


2Aj^A2 


2 FA, 


In this three-ammeter method the non-inductive parallel 
branch is equivalent to an added current^ while in the three- 
voltmeter method the non-inductive series resistance means an 
added voltage. Both methods, therefore, require the supply of 
practically twice as much power as that needed for the circuit 
under test. Further, the losses in the ammeters, voltmeter, and 
wattmeter may cause serious errors in the results if the currents 
are small. For the above reasons, no one would use either 
method for measuring power if a wattmeter was available, 
except from a purely scientific interest. While the power 
absorbed and phase difference may be calculated*in each method 
from the vector diagram, constructed for each set of readings, 


it would usually be obtained from the respective formulae. 

Apparatus. —That indicated in Fig. 141, where D is an adjust¬ 
able source of alternating current, preferably a motor-driven 
alternator, the frequency, current and voltage of which can be 






ELEGTEICAL ENGINEERING TESTING 


387 


varied independently. A voltmeter V is connected across the 
parallel combination, and a wattmeter inserted so as to measure 
the total watts absorbed in the parallel combination merely for 
the comparison of the three-ammeter and wattmeter methods. 

Observations, —(1) Connect up as in Fig. 141, levelling and 
adjusting to zero such of the instruments as need it. 

(2) See that all lubricating arrangements are in operation 
on starting up. 

(3) Adjust the speed to get maximum frequency, and also the 
voltage V of the alternator (by varying its excitation) so as to 
send the maximum safe current A 2 through PQ, and note in 
rapid succession the readings of 

Note. —If possible adjust the non-inductive resistance r so 
as to obtain the conditions for maximum accuracy (other things 
being the same) of 

(4) Repeat (3) for about six different frequencies between the 
maximum and minimum values possible, using the same current 
^2 in each case by suitably adjusting the speed and excitation 
of the alternator. 

(5) Repeat (3) at constant maximum frequency for about six 
different values of current A 2 between the maximum and 
minimum values possible by varying the excitation, and tabulate 
your results as follows— 


Form of Inductive Circuit PQ, tested . , . 
,, Non.-Ind. Resistance r used . . . 


c 

0) 

S 

cr 

£ 

Voltage V. 

Currents. 

Power in Watts absorbed 
in 

Power 
Factor in 

From 

Math. Tables 

Res. of 

1 Coetf. of Self-Ind. of (PQ) 

j_^R tan 

! 

CO 

a 

c 

II 

a> 

0 

0 

C 

P 

c« 

0 

eS 

0 

rt 

m* 

R. + 

© V. 

^ n 

S 

1 Impedance of Parallel Combination (- /.(i. | 

A. 

Ai. 

A 2 . 

PQ 

-t-r. 

r. 

PQ. 

(PQ + r) 
By Calcu¬ 
lation. 

PQ 

+ r. 

PQ. 

Angle of 
Lag in 

rH 

a 

ci 

H 

r. 

PQ. 

Wattmeter IF. 

rH 

II 

fH 

S 

*05 

1 

M 

1 

•1 

1-1 

l<M 

II 

Cl 

8 

Cl 

8 

-f 

8 

II 

8 

Cf} 

0 

II 

*C1 

1 

C« 

+ 

M 

3 

II 

0 

**C1 

1 rH 

+ 

C4 

1 

<S> 

CO 

0 

0 

*01 

1 

1" 
M 

II 

rH 

CO 

0 

0 

6°. 

01°. 

II 

(- 

R. 






1 1 










' 


























































388 


ELECTRICAL ENGINEERING TESTING 


(G) Plot curves having values oi A^ as abscissae with values of 

F, cos $ 1 , L, ~ and respectively, for the inductive portion RQ 
^2 

as ordinates. 

(7) Draw the vector diagram (Fig. 142), for the maximum 
current used. 

(8) Compare the value of A with the algebraical sum A-^-\- A^] 
also IF with w. 

Inferences.—What can you infer from the results of the test 
and from the curves? 

(135) Measurement of Power in Three- 
Phase Alternating Current Circuits. 

Introduction. —The measurement of the electrical power 
absorbed in a 3-phase alternating current circuit might well at 
first sight appear somewhat complicated. In reality, however, it 
is very little more so than in the case of single-phase circuits 
and the actual extent to which it is, depends mainly on the 
nature of the circuit in which the measurement is being made. 
It has already been seen that the non-inductive Wattmeter 
forms the best means of obtaining the true fower absorbed in 
a single-phase circuit, but with multiphase circuits usually, 
though not always, two such instruments are necessary. 

The object consequently of the present investigation is not 
only to state the methods of measuring, but also to prove the 
truth of them under the several distinctive conditions met with 
in practice. 

The circuit in which the power has to be measured may be of 
the type shown at {a) Fig. 143, which is known as the star or 
0 ][)en form, or of the type shown at (6) Fig. 143, known as the 
mesh or closed form, (c) represents the circuit containing the 
measuring instruments, which may be connected to either (a) or 
(6) arrangements at will, E being the source of polyphase supply. 

Now let A-^ A 2 A^ and ^2 ^3 be the -y/ mean square values of the 
currents fiowing in the mains and branches respectively for Fig. 
143(6 and c), also F^ F 2 Fg and Vg the same values of voltages 
across the mains and branches respectively for Fig. 143 (6 and c); 
then if the mains are equally loaded we have—• 



ELEOTlilGAL ENGINEERING TESTING 


389 


whence = 2^2 sin. 60° = and A = ^3(X, since the mains 

are equally loaded and the load non-inductive. 

For Fig. 143 (a and c) we have, if ^^ = ^2 = ^3 ^1 = ^2 = ^3’ 

that = A2 = ct2 and A^ = a^, and since V will now lag 30“ in 
phase behind r, in each main and the corresponding branch 
circuit, we have V = 2 r sin. 60“ = providing the load is non- 
inductive. 

Circuits equally loaded and non-inductive.— Here if each 
main carries the same current A, and if the pressure between 




b c 

Fig. 143. 


each pair of mains = V, then the True Power absorbed in a non 
inductive load, Fig. (b) 

W=3av = 3 vA^ = \'3AV Watts, 

V 3 

True Power absorbed in a non-inductive load, Fig. (a) 

IT = 3«r= 3A—j^ — sj3 AV Watts. 

If, however, the load is inductive, then if ^ = angle of phase 
difference between voltage and current, we have, as in the case 
of single-phase work, that for equal load the True Power absorbed 
in the inductive load, Figs, {a) or {h) 

1F= J 3 A V cos. 6 Watts. 

This latter can best bo obtained by means of the non-inductive 
Wattmeter for each of the two following conditions met with in 
practice. 

Circuits equally loaded and inductive—One Wattmeter 
ONLY needed to obtain the tvue powev. Assuming this to be TFj, 
Fig. 143(ca) or (c h), then with the thick coil in any main (^3 say, 
as shown) note the Wattmeter reading with its fine coil on" 



















390 


ELECTRICAL ENGINEERING TESTING 


to main A^, and the reading (w^) with it on to main A^ imme¬ 
diately after, tlien the True Power absorbed in the equally loaded 
inductive circuit W — w-^ ± where both tv^ and will vary 
with load and power factor. 

The reason why or trg alone does not give the power of 
the circuit is because A^ and are not in phase, even in a non- 
inductive circuit, but differ in phase by an angle = 30° d: 
for both star and mesh connections. Therefore will read the 


a/s 

product AqV^^ cos 30 = — ^ 3^^2 unity power factor. Thus 


we see that u\ = -dgFg cos ( 30 ° - f - ^), and W2 = ^2^3 
— ^), and the sum of these after expansion = 2^3 Fg (cos 30° 

cos <^) = \/3^3F2 cos (/) = iCgj which is the true power in 
the circuit. If now the load is so highly inductive that (/> 
exceeds 60°, i. e. the power factor cos (/> is less than 0‘5, then 
cos (30 -}- <^) becomes — and the wattmeter will reverse for 
one of its readings or which must therefore be considered 
as — since the volt-coil connection must be reversed to get a 
scale reading. 

. •. the total power Tr= — ^^g = 2 ^ 3 Fg sin 30° sin 
= 2 ^ 13^2 X i sin </>= Fj X J 3 sin and jfa F ^<3 sin 4 , 

==A 2 sin (}> = the wattless or idle line current. 

The above results will be readily understood by a reference to 
Fig. 144 (corresponding to Fig. 143, a and c), in which OA, OB^ 
00 , represents the voltages across the respective star stator 
phase windings in magnitude and phase] difference (= 120 °). 
AC, OB, BA = voltages F^, Fg, F 3 between mains in relative 
magnitude and phase (= 120 °). 

Then obviously V-^~ AC — J^OA = \/3(9(7, 

also Fg = (7i?= JWC = IfiOB, 

and V^ — BA — '^J'iOB — J?>OA. 

Now, since the three phase-windings are inductive we can 
draw three equal lines, Oa, Oh, Oc, to represent the currents in 
them lagging in phase by equal angles 0 behind their respective 
voltages OA, OB, OC. 

Then the current Oa in its phase winding differs in phase from 
the voltage AB (= F 3 ) by an angle aDA = 6 — 30°, and the 



ELEGTBICAL ENGINEEBING TESTING 


39i 


current Ob in its phase winding differs in phase from the voltage 
BG ( = Fg) by an angle OcB = ^ -f- 30°. 

Now, if the current coil of wattmeter is in the circuit of 
OC, and hence of main 3, with its volt coil across CB (mains 
3 and 1), it will carry the current Oc at a voltage Similarly 
the current coil of wattmeter in the circuit of OA, and hence 



of main 2 , will carry the current Oa with its volt-coil across AB 
(mains 2 and 1 ) at a voltage F 3 . 

Then il\ = -dgFg cos {6 -f- 30°), 

= ^ 3 F 2 (cos 6 cos 30° — sin 0 sin 30°), 
and iv .2 — A cos {0 — 30°) 

= A^V^ (cos 9 cos 30° sin 6 sin 30°). 
total power of the circuit W =■ %V 2 = ^AV (cos 6 

cos 30°) — J'^AV cos 9, on the assumption that = A^ 

= A, and F^ = Fg = F 3 = F, which should be the case. 

By adding and subtracting the values of and first given 
we have 


u\ 


Wc 


u\ + U’2 

we have the power factor 

a -|- 1 


1 

—T= tan 9 j and putting —^ = a 
V 3 u’l 


cos 9 = 


2 Va2- 


Vl + 3(^ 




(see p. 399), 


+ ^2 


when ^ = 0 the values of w-^ and are equal, and each 


= hJ'^AV cos 9. 

As 9 increases, u\ decreases and increases, when 9 = 30° 
the value of t^^i = - 2 -^f 3 F 2 or J^F, and of w<^ = A^V^ or AV; 
when 9 — 60° the value of = 0 , and of 

















392 


ELECTRICAL ENGINEERING TESTING 


Since in this case the current in the series coil of differs 
90° in phase from that in its pressure coil, any further increase 
in 0 will make negative and reverse its deflection, so that the 
connections of one of its coils must be interchanged ih order to 
bring the deflection on to the scale again. 

Hence in measuring the power of any inductive 3-phase 
circuit by either 1 or 2 wattmeters the total power = A: ^' 2 ’ 
•?, e. if one of the readings reverses, snhsiract the smaller reading 
from the larger one to obtain the total power. 

Circuits unequally loaded and inductive— Two Watt¬ 
meters ONLY needed to obtain the true 2 ')Ower. Assuming the 
Wattmeters to have their thick coils in any two mains, as shown 
in Fig. 143 c(« or 6 ), then True Power absorbed 1F= TFg- 

Hence, when merely the true power in Watts only is required 
in a three-phase circuit, whether of the star or mesh type, one or 
two WatLneters are required according to whether the circuits 
are equally or unequally loaded respectively. Also when such a 
three-phase circuit is both equally loaded and non-inductive the 
true power in Watts is given by the product J'd x amps, in one 
main x volts, across any pair of mains. (See p. 395 seq.). 

Apparatus.— Source of three-phase alternating current (A) and 
circuit of variable nature to experiment upon (a and 5, Fig. 143). 

Two Wattmeters and ; three Siemens dynamometers or 
Parr direct reading dynamometer ammeters A-^ A^ A ^; three 
electrostatic or hot-wire voltmeters V 2 F 3 . 

Note. —It must be remembered that for any specific measure¬ 
ment, the foregoing rules, and the instruments they entail, can 
be at once used without reference to the following test, which 
is devised solely in order to prove these rules. 

Observations. —( 1 ) Connect up as in Fig. 143 {a and c), and 
adjust the instruments to zero, levelling them if necessary. 

(2) With the load non-inductive and the circuits equally loaded, 
take the readings of all-the instruments for five or six different 
loads, noting the Wattmeter reading when placing the fine coil 
of, say, IFj successively on to Aj and A^ mains at each load, 

(3) With an inductive load and circuits equally loaded, take the 
leadings of all the instruments for five or six loads, placing the 
fine coil of, say, IFj successively on to A j and Ag mains at each load 
and noting its reading at each. Tabulate your results as follows— 


ELEGTBICAL ENGINEERING TESTING 


393 


.T. 

to 4:3 

.9 ^ 

Wattmeters. 

Voltmeters. 

Ammeters. 

ir = 

JFi 4- IF 2 . 

JF = 

2 IFi. 

IF = 
s/3JF. 

6 

S ® s- 

Wi. 

JV2. 

Fi. 

^2. 


A- 

^2- 

^3- 















Inferences.— State very clearly all that can be inferred from 
your experimental results. 


Measurement of Power in Two-Phase 
Alternating Current Circuits. 

Introduction. —Two distinct forms of circuits are met with in 
the distribution of electrical energy by means of two-phase alter¬ 
nating currents of electricity. 

The first entails the use of four wires, forming two circuits 
completely independent of one another, one to each phase. Since 
this requires four wires it is usually employed in short distance 
transmissions. 

The second entails the use of only three main wires, and is 
therefore more economical in first outlay of copper than the 
above. It will therefore be at once obvious that the measure¬ 
ment of power in two-phase alternating current circuits will be 
made in more than one way, depending on the form and nature 
of the circuit in question. We will now deal with such measure¬ 
ments in the case of each possible condition. 

Two-Phase Circuits of the 4-wire Form. 

Here two cases are possible according to whether the circuits 
are carrying non-inductive loads, such as incandescent lamps, or 
inductive loads, such as two-phase motors or transformers, etc. 

Non-inductive load .— The product of the amperes and volts in 
each circuit, obtained in the usual way, when added together 
gives the true power delivered from the generator; and if the 
two circuits are equally loaded, tivice the product for one circuit 
gives the Total True Powder. 

Inductive load .—Owing to the lag in phase between the current 
and voltage in each circuit, two non-inductive Wattmeters are 
necessary, one in each circuit, connected up in the ordinary way 






























394 


ELECTRICAL ENGINEERING TESTING 


as in single-phase circuits. 



Then the Total True Power delivered 
hy the generator = sum of the two 
Wattmeter readings. 

If the two circuits are equally 
loaded, as should be the case when 
supplying such as two-phase motors, 
then twice the reading of one Watt¬ 
meter gives the Total True Power, 
and only one such instrument is then 
necessary. 


Two-Piiase Circuits of the 3-wire Form. 

Here also there are two or three cases depending on whether 
the circuits are inductive or otherwise. 

Equally loaded non-inductive sections. —Total True Power ab¬ 
sorbed = twice the product of the current in one outer main and 
the voltage across the section. 

Equally loaded inductive sections. —Total True Power absorbed 
= twice the reading of a Wattmeter connected with its thick 
coil in series with either outer main, and its thin coil connected 
to the centre or larger main which is common to both outers. 

Unequally loaded inductive sections. —Total True Power ab¬ 
sorbed = sum of the two readings of the Wattmeters connected 
with their thick coils in the outers respectively, and their thin 
coils connected to the common centre wire as shown in Fig. 
145. This last case would be the one met with when the circuit 
was partly a lighting and partly a power one, running two-phase 
motors. 

Where the reader may not be quite conversant with the pre¬ 
ceding methods of measuring power in two-phase alternating 
current circuits, a most useful experiment will be to prove the 
above statements in much the same manner as was set forth in 
the preceding test on three-phase measurements of power, only 
three or four ammeters and voltmeters with the two Watt¬ 
meters TTj and and the variable two-phase rheostat being 
required. 















ELECTRICAL ENGINEERING TESTING 


395 


Measurement of Power in Polyphase 
Alternating Current Circuits. 

As a summary, with some additions, to the methods as given 
on pages 388-394, the principal arrangements of wattmeters 
employed for measuring the true power in different kinds of 
alternating current circuits commonly met with in practice, are 
given here in diagrammatic form. 


JINGLE PHASE 



'Wj =JE C cos (& 

W-Wj 

Fig. 14bV 


TWO PHASE (BALANCED) 
INDEPENDENT OR COMMON RETURN 



'W, =E C cos ^ 
W = 2 ^ 

Fig. 148. 


TWO PHASE (UNBALANCED) 

INDEPENDENT OR COMMON RETURN 



4\J-E,C,COS 0 '^EjCgCOS 0 


Fig. 147. 


THREE PHASE (UNBALANCED) 

USING THREE INSTRUMENTS 



'\VjECfCQS(p "WJEC^cos^ T^EC^cos0 
Fig. 149. 


IF = the total watts in the system. 

IFj, TTg, TTg = „ readings of the various wattmeters. 

E, Ag = „ voltages „ „ ,, sections. 

(7, C'l, Cg, Cg = ,, currents in „ „ „ 



































I 


393 ELEGTEICAL ENGINEEBING TESTING 


r, r^, = the non-inductive resistances in series with the 

fine wire coils of the wattmeters. 

<fi, (f)i, <f) 2 , — >} angles of phase difference between currents 

and voltages. 


The diagrams and deductions 
principle clearly enough. 


THREE PHASE (UNBALANCED) 
USING TWO INSTRUMENTS 



y^-\3EC,cos(e,*<p) Tl5-V3EC^cos(% 
Fig 15C. 


accompanying each explain the 


THREE PHASE (BALANCED) 
NEUTRAL POINT AVAILABLE 



l^=EC^COS 

W=3Wj 

Fio.151. 


THREE PHASE (BALANCED) 
NEUTRAL POINT NOT AVAILABLE 



Fig. 152. 


THREE PHASE (BALANCED) 



”\^=ECcos <p 


V3EC^osr^-y ) 

=vsec^in 0 

= V3 X WATTLESS POWER 
PER PHASE 


Fig. 153. 


In Fig. 150, if the power factor of the system is less than 0-5, 
one of the wattmeters will read negatively and the connections 
of its fine wire circuit will have to be interchanged in order to 
obtain deflections on the scale. In this case the difference of the 
two wattmeter readings gives the total i)ower. 

In Fig. 152, the resistances r^ — r^ = (r + fine wire coil), but 
an artificial neutral point can be formed by lamps without the 
expense of the resistances 9*^, r^* 

In Fig. 156, unless the resistance of the fixed current coil is 





























ELECTRICAL ENGINEERING TESTING 


397 


small compared with the resistance of the phase in series with 
which it is connected, its insertion will throw out the balance of 
a mesh system and 3 will not = the total true power. The 

THREE PHASE (BALANCED) THREE "PHASE (UNBALANCED) 

WITH TRANSFORMERS 



'Ce^cos (f> T^-Cye^cos ^ 

'W,‘/Ocos30"^700>icos <f» W^1Q-I00cos30*cos <f> 
'^SGGcos (p 'SCGcos ^ 

V- w- 2 

Fia, 154. Fia. 155. 

arrangement in Fig. 150, or if the system is balanced one watt¬ 
meter with a two-way key for connecting one end of the fine 


Thrtt Phase (Batanced) 


Ct 



' w ^ 3 w, 
w, = f-^ccosp 

Fig. 156, 

wire coil in quick succession to the remaining two mains, is 
much to be preferred. 

Fig. 155 shows a method of connecting two wattmeters in 
three-phase high tension mains ABC using current and pressure 























398 


ELECT RIGA L ENGINEERING TESTING 


transformers. F and K are two 20 to 1 series transformers, 
giving a secondary current of 5 amps, at full load; while H is 
a 10 to 1 series transformer, giving a secondary current of 
1 amp. at full load. T and S are each 100 to 1 pressure trans¬ 
formers, with 10,000 volts on the primaries. It can be shown 



that each wattmeter indicates 866 kw. at full load, the total 
power of the circuit being 1732 kw. 

Note. —The above methods are equally applicable to measuring 
the output of a generator or input into a motor or rheostats. 

The measurement of power factor in alternating current 
circuits can be made by moans of power factor indicating 


POWER FACTOR 





























































































ELECTRICAL ENGINEERING TESTING 


399 


instruments, or by the method described on page 388, in the case 
of three-phase circuits. Another method is shown in Fig. 153 
for three-phase circuits in which a wattmeter Wp connected as 
shown indicates the wattless power in a phase, or the power 
factor, if the scale be suitably graduated. The instrument in 
this case has a central zero and deflects to one side or the other 
according to whether the current lags or leads with respect to 
voltage. 

As with such an arrangement, a considerable P.D. will exist 
between the fixed and moving coils, it can only be recommended 
for the lower voltages. 

Another and safer method can be employed with balanced 
three-phase circuits using one wattmeter in one main and a 
two-way key for connecting one end of its fine wire circuit in 
quick succession to the remaining two mains. 

If and are the two deflections so obtained, then the 

1 


Power factor of the circuit = 



+ 3 

d^i 


Correcting Factor for Wattmeters. —Considering the most 
common form, namely the electro-dynamometer type, used in 
practice, it is well known that the current through the moving 
coil should be exactly in phase with the P.D. at its terminals 
for the instrument to read true loatts correctly. It is therefore 
both interesting and important to know the magnitude of the 
error introduced into the reading of the wattmeter and the 
correcting factor to be applied to obtain true watts when the 
current and pressure in the fine wire coil are not in phase due 
to the coil possessing inductance, which it must necessarily have 
to some small extent. The following considerations are quite 
general, and assume that the current and voltage are sine 
functions— 

Let G = the maximum current in the fixed coil, 

I = the current (at time t) in the fixed coil, 

E sin (Jit == potential difiference between the mains (at time t)y 
cf) = angle of lag of the current in the mains, 
i = the current (at time t) in the moving coil of self- 
induction L and total ohmic resistance i?, 
including any resistance in series with it. 





400 


ELEGTIUGAL ENGINEERING TESTING 


(o = 27r X frequency of the supply. 

Tlieii the relations given in Fig. 157 ^ can be shown to hold good. 

The relations apply to all types of wattmeters if <^, L and w, 
the only quantities which vary with the nature of the load and 
type of wattmeter, are known. 

* When ^ = 90° the multiplying factor becomes zero, and the 
reading of the wattmeter is zero, since there is no force between 
the coils carrying currents which differ in phase by 90*^. The 
curves (Fig. 157) can be used as follows—Suppose we know that 
L = 0 02 henry, R = 628 ohms, frequency = 50 per sec., and 

the power factor = 0’5. Then = O'Ol. 

J. V 

Hence curve G is to be used. Now the horizontal line through 
0 5 on the power factor scale cuts the power factor curve cos. ^ 
at a point, the vertical line through which passes through 
cfi = 60° and cuts curve C at 0*98, which is the correcting factor 
of the wattmeter. 

Fundamental Considerations Relating to 
Alternating Current Static Transformers. 

General Remarks. —Before considering actual methods of 
testing static transformers, the importance of which, in alternat¬ 
ing current systems of distribution of electrical energy, arises 
from the ease with which a small current at high pressure can 
be converted to a large current at low pressure or vice versa 
by such an appliance and with very little loss, some introductory 
remarks are considered desirable. 

There are a great many different forms and ways of building 
the kind of transformer in question, but they all come under one 
or other of two main heads, namely— 

{a) Those with closed magnetic circuits in which the magnetic 
induction or lines of force are contained solely, or nearly so, in 
iron. 

(b) Those with open magnetic circuits in which the lines of 
force run partly in the iron core of the transformer, and partly 
in the air through which they complete their path. This type, 

1 Taken, together with Figs. 104 and 146-155 from a paper on “The 
Measurement of power in alternating current circuits,” by P. Hamilton, 
Proe, Inst. C.E., vol. cliv, 1902-1903, by kind permission of the Author and 
Inst. C.E. 




ELEGTRIGAL ENGINEERING TESTING 


401 


however, has now become practically obsolete. In either case 
(a and b) the iron core is surrounded by or wound with two dis¬ 
tinct and separate coils of insulated copper wire termed the 
'primary and secondary. In all cases the former is the coil con¬ 
nected to the source of supply, while the latter has induced in it 
an E.M.F. which supplies current to some separate circuit, usually 
at quite a different E.M.F. to that acting on the primary. 

The primary may be either the high tension (pressure) coil or 
the low, according as to whether the transformer is used as a 
step-down or step-up appliance respectively. Hence to avoid 
confusion, the primary will always be that coil which is connected 
to the source of supply, whether this be high or low tension. 

It may now be well to consider certain phrases and quantities 
met with in static transformers, and which appear in testing 
work on them. Transformers with “closed” magnetic circuits 
only need be considered, the “open” magnetic circuit type 
not having been made for many years. The induced secondary 
voltage is evaluated as follows—• 

Let N = total magnetic flux threading the secondary 

winding of Tg turns, 

f = periodicity of the primary supply-current, and 
hence of this flux, 

Ep and Eg = maximum values of E.M.F.s at the terminals of 

primary and secondary. 

Now since in one period of the current wave, the current and 
hence the flux varies from 0 —max., max. — 0 , then reverses and 


again varies from 0 —max. and then max.— 0 , the average rate 
of change in the flux = iN lines per cycle or period, and the 
average change = lines per sec. Therefore the average 

E.M.F. induced per turn therefore the average E.M.F. 


induced in the secondary winding of Tg turns = .£’5 = 


1U8 

volts. Since the virtual E.M.F. = average E.M.F. X form 
factor of the voltage wave, the virtual or R.M.S. E.M.F. 

E. — ^ X I'l l Nflg _ 4 44iV^/7 g yQp-g ’vvhere 1*11 is the value 

- -^Q8 108 

of the form factor of a sinusoidal wave. There will also be an 


D D 






402 


ELECTRICAL ENGINEERING TESTING 


induced E.M.F. due to self-induction in the primary winding of 
Tp turns, and since the same flux thi’eads this also, this back 

E M.F. of self-induction must = ^ - volts. On open 

10 ^ 


secondary circuit the primary supply pressure only exceeds this 
back E.M.F. by a very small amount, namely, that sufficient 
to force the energy current through the resistance of the 
primary winding and provide the necessary magnetizing current 
for producing the flux in the core. We therefore have the 
following important relation, namely— 

Ep iAiN/Tp ^ 10^ , 

Es ^'UNfTs W ^ 

very approximately, which is called the voltage ratio of conversion 
or ratio of transformation. 

li Ap and As are the currents flowing in the primary and 
secondary having resistance Rp and Rg, then the ohmic drop of 
voltage in each is ApRp and AgRs respectively, and the core 
flux is produced by an effective voltage Ep — ApRp^ where the 
bar over the expression indicates that it is a vectorial—and 
not an algebraical—difference, the primary supply E.M.F., Ep^ 
and energy voltage, ApRp, not being in phase as indicated in 
Fig. 158. 


The no-load secondary induced voltage will 
therefore 

T, 


=z[Ep — ApRp)^ volts, 

and the secondary voltage on load 

T 

-L a 


— {Ep ApRp^— [- AgRg volts. 

-L P 

When the secondary circuit is open, the total 
loss occurring in the transformer is called the 
open-circuit loss, and the current flowing in the 
primary is called the no load primary current. 

The open-circuit loss is made up of the copper 
loss due to the no-load current flowing in the 
primary winding, and which is usually very small 
with the remaining loss due to eddy currents and 
hysteresis which are termed the iron core losses. 



Fio. 158. 

compared 

magnetic 











ELECTRICAL ENGINEERING TESTING 


403 


The no-load primary current, such as would be indicated by 
an ammeter, consists of two components in quadrature, namely, 
(a) the true magnetizing component, which being an idle or 
wattless current lags 90° behind the supply voltage, and {b) the 
energy or load component in phase with the supply voltage, and 
overcoming the above open-circuit losses due to eddy currents, 
hysteresis, and copper loss. 

These three currents can therefore be represented by a right- 
angled triangle such as Fig. 158, in which BD would be the no-load 
current, BC the energy component, and CD the magnetizing 
component. Thus, since BD = JBG'^ GD^, we see that the 
no-load current = -y/ (energy current)^ (magnetizing current)^ 
= sJAg^ + A^\ and this no-load current would be in quadrature 
with the supply volts, except for the energy current, which 
makes the phase difference slightly less than 90°. 

The magnetization or core flux, being directly proportional to 
the supply voltage at constant frequency, is constant at all 
secondary loads with a constant voltage supply, and hence the 
iron losses are constant at all loads. Further, since we have seen 


that^ = it follows that ^ = i. e. the primary 

Eg Ig Ap Ig hjg 

and secondary currents are inversely oc to the voltages. 

The measurements of current, voltage, and power in tests 
connected, not only with transformers, but also with alternating 
currents generally, should be made with instruments possessing 
practically no self-induction and little or no iron. The best 
results will be obtained when employing electrostatic, hot-wire, 
and dynamometer instruments, for such measure the ^mean sq. 
values of pressure and current and are independent of the varia¬ 
tions of frequency. If a circuit supplied with alternating current 
is non-inductive, as for example a bank of electric incandescent 
lamps run off the secondary of a transformer, then the ^mean sq. 
values of the amperes x that of the volts = the true or mean 
'power in Watts taken up by that circuit or bank of lamps. 

If, however, the circuit is inductive this product (amps, x 
volts) gives what is called the apparent power in Watts absorbed, 
which is in all cases greater than the true power. This would be 
the case if we tried to measure the power given to the primary 









404 


ELECTRICAL ENGINEERING TESTING 


of a transformer, which is always very inductive. Recourse must 
in such cases be had to the so-called non-inductive Wattmeter^ the 
fine wire coil of which must have as few a number of fine wire 
turns as will give the requisite sensibility. Such an instrument 
will measure the actual or true mean power given to any circuit, 
however inductive it is, and no difficulty presents itself in the 
use of the Wattmeter on a low tension circuit. If, however, the 
power absorbed in a high tension circuit is required, then a special 
arrangement of Wattmeter is needed (see p. 42). It is much 
better, however, to have all measuring instruments on the low 
tension circuit, and this can be accomplished by employing one 
of the double conversion methods given in the following pages, 
a course almost always possible in works and central stations in 
which two similar transformers as regards size and output can 
generally be obtained. 

Another method of measuring the power given to or developed 
by a transformer is the 3-voltmeter one, and in the case of the 
primary circuit, a non-inductive resistance of such a known value 
is placed in series with this coil, that the P.D. across its ter¬ 
minals = that across the primary coil, or preferably as nearly so 
as possible, as this gives maximum accuracy. The method con¬ 
sequently has the somewhat serious disadvantage that the E.M.F. 
of the supply has to be double that required for the primary 
alone, which would in the majority of cases preclude its use. 
Then again a small error in observation may cause a large error 
in the results. 


(136) The Effect on the No-Load Voltage 
Ratio, Current, and Watts of a Trans¬ 
former, of Change of Primary Supply 
Voltage and Frequency. (Magnetization 
Curve or Open Circuit Characteristic.) 

Introduction.—The present investigation is a very important 
one, in that, amongst other results, it gives the relation between 
primary terminal voltage (a to core flux at constant frequency) 
and magnetizing current, and which is termed the “ open-circuit 
characteristic” or “magnetization curve” of the transformer. 


ELECTRICAL ENGINEERING TESTING 


405 


The voltage used for the relation should, strictly speaking, be 
that of the back E.M.F. of self-induction, and therefore the 
vectorial difference Vp — ABp‘, but as both A and Rp are small, 
their product is negligibly small compared with Vp, and can be 
neglected. Even with the special low-loss iron now used in 
transformer cores, these are seldom worked at magnetic induc¬ 
tion densities outside the limits, 3500 to 7500 lines per sq. cm.^ 
in order to minimize the power (due to the iron-loss or energy- 
current component) absorbed in magnetization, which is con¬ 



verted into heat in the core. For this reason the “knee” of 
the curve, which corresponds to about 15,000 to 17,000 lines 
per sq. cm., and is all-important in the design of D.C. apparatus, 
is never reached in the magnetization curve of a transformer. 

Further, since the core loss is obtained in this test and is 
well known to be practically constant at all loads, it follows 
that, knowing the resistance of the windings, and hence copper 
losses (C^R) in P and S at any load current, the efficiency can 
be predetermined at all loads. 

The test also shows that both the no-load current and watts 
decrease as the frequency increases, and hence that higher 
frequencies reduce the size of core and cost of manufacture 
for a given output. 

Apparatus. —Transformer under test, of which P is the primary 
and S the secondary; low-reading wattmeter W ; voltmeters 
VpVs] switch K frequency meter low-reading ammeter A\ 
source of supply E, preferably a motor-driven alternator, the 
speed and excitation of which is variable over a wide range. 

Observations. —With Variable Voltage Supply at Constant Fre¬ 
quency. 

0) Connect up as in Fig. 159, levelling and adjusting such 
instruments to zero as need it, the terminals it of the high 
















406 


ELECTRICAL ENGINEERING TESTING 


tension winding used as the secondary being open-circuited as 
shown. 

(2) With the frequency adjusted to the norma) value for tlie 
transformer, and the field regulator of the alternator full in, 
close K and take simultaneous reading of Vp, Vg, F, IV and A at 
each of some ten different values of F/., rising by about equal in¬ 
crements from the lowest readable values to not exceeding 20 % 
above normal, by adjustment of field regulation or otherwise, and 
at constant normal frequency. 

(3) With Variable Frequency Su^yply at Constarit Voltage. 

With the voltage adjusted to the normal value for the trans¬ 
former, take simultaneous readings of Vp, Vg, F, W and A at 
each of some ten different values of F, rising by about equal 
increments between the lowest and highest values convenient, at 
constant voltage Vp. 

(4) Measure the ohmic resistances of the primary and second¬ 
ary windings F and S; that of P by either (a) the ammeter- 
voltmeter method (p. 86), using Ohm’s law and a direct current 
supply for E, taking care to connect a suitable main current 
variable rheostat in circuit between E and P; or (b) the com¬ 
parative deflection method (p. 84). The resistance of E may 
be obtained by either (c) method (a) above mentioned, or 
(d) a Wheatstone bridge. In the case of the ammeter-voltmeter 
method, the voltmeter must be connected to the actual terminals 
TT or tt of the windings. 

Tabulate all your results as follows—- 


Transformer: No. . 
Normal output =. 
Secondary : volts ; 

Primary ; volts = , 


Type . . . 

. . KVA. Frequency = per sec. 

:. . . Amps. =. . . Uesistance lig = 


Amps. = . . . 


Resi.stance Ap =, 


C 

<x> 

O' 


Voltacces 


s 

‘C 

Ph 


b 

oi 

a 

o 

o 

o 

OQ 


c3 . 

K ^ 

o 

> 


tD 

ft 

S 

•< 


w 

Li 




O 

^ . 

CO 




ft: 

a a, 


fe II 

c3 

ft 

CD 

!■©- 

ft 

<5 

H 

s 

CD 


• 



bo 

a 

<t-( 

o 

® 

W) 

n 


c 

o 

ft 

C & 

g II 

w 


Maker . . . 
Voltage ratio = 
ohms @ . . . 
Turns Tg =, 
. ohms @ . . . 
Turns Tp = 


°C. 

°C. 




bo ” 
c a 

:bs 

I ^ 

S)S 

03 O 

® g II 

® t S 

-O p ^ 






X 

rH 

S 

o 

u 

o 

O 

































ELEGTBICAL ENGINEElilNG TESTING 


407 


(5) From obs. 2 plot the “ open-circuit characteristic ” (other¬ 
wise known as the “magnetization curve”) of the transformer 
having values of Vp as ordinates, with magnetizing current 
as absciss86. 

Also curves having the same scale values of Vp as ordinates, 
with (a) total no-load current A; (h) energy current compo¬ 
nent Ap (c) no-load watts W (practically all iron-core losses) 
(d) voltage ratio F^/Fp, as abscissjc, in each case. From obs. 3 
plot on another curve-sheet curves having values of frequency 
/ as ordinates, with A, Ap, Am, W and F^/Fp, respectively, as 
abscissae. 

Inferences. —From a study of the table of results and shape 
of curves state clearly all that can be deduced. 


(137) Measurement of Copper Losses in a 
Transformer (by the Short Circuit Test). 

Introduction. —The total internal loss IF in any static trans¬ 
former is made up of the iron loss TF^ due to eddy currents and 
magnetic hysteresis in the iron core, together with the copper 
loss Wq due to the currents Ap and A^ in the primary and 
secondary windings of resistances Ep and 7?^. 

Then TF^, = Ap^Ep -f A^^E^ and 1F= JFj -f TF^. 

Knowing Ep and Eg, the copper loss TF^ can be calculated for 
any or a series of measured load currents, but the value so found 
may differ considerably from the actual working or effective 
value, owing to the eddy current and “ skin effect ” present with 
the larger sizes of conductor, when carrying alternating current, 
causing an apparent increase in the resistances Ep and Es. 
The present test, comprising the direct measurement of the total 
copper loss, would therefore appear to be a means of obtaining 
it under working conditions, and hence more accurately than by 
calculation. 

Another source of error may, however, now creep in, for the 
wattmeter necessary for measuring the loss must obviously be 
a low-reading one, and have a current capacity equal to that of 
full load for the winding chosen as primary, while its pressure 
coil will be subject to a small fraction of what would probably 
be its normal pressure (a condition introducing an error in its 


408 


ELEGTBIGAL ENGINEERING TESTING 


indication) unless, of course, the wattmeter is a specially designed 
one for low pressure. The small applied voltage necessary for 
keeping the short-circuit current within safe limits, will produce 
a very small induction, and therefore loss due to magnetization 
of the core. This last named may be negligibly small, when the 
wattmeter will indicate the copper loss only. If the iron loss 
is not so small, the wattmeter will give a reading at the applied 
voltage of short circuit when the secondary is open-circuited, 
and this reading must be subtracted from all of its indications 
on short-circuited secondary. Further, care must be taken that 
the wattmeter reading does not include any losses in connecting 
or short-circuiting cable. If it does, the loss in such must be 
separately calculated from their measured resistance and each 
current, and deducted from the reading. 

If Tp and I's = fhe number of primary and secondary turns 
respectively, and Fp = a small supply voltage applied to the 
primary in order to send full-load current Ap through it with 
secondary short-circuited, then the total resistance “drop” 

= Ap(^Bp -j- Rs ^ volts. 

From the values of this “ drop ” and Vp the characteristic 
triangle of the transformer can be drawn and the leakage drop 
determined.! 

Apparatus. —That for test No. 136, excepting that IF and 
I'p must now both be low-reading instruments, while Vs is 
replaced by a low-resistance ammeter Ag for short-circuiting the 
terminals tt of the secondary winding the range being large 
enough to indicate at least full-load current of that winding. 

Observations. —(1) Connect up as in Fig. 159, with the 
ammeter Ag across (tt) and the pressure circuit of IF across TT, 
in order to eliminate errors due to including in the reading of 
IF any copper loss in the primary connecting cables. Level and 
adjust to zero any instruments which need it. 

(2) With an efficient short circuit of S the primary P will 
practically constitute a metallic resistance and require, by Ohm’s 
law, probably only two or three volts or so to be applied to it 
in order to obtain full-load current through it. 

1 Vide, “The Testing of Transformers,” by MoriL utid Ijster (Journal 
I.E.E.. vol. 37, p. 264, 1906). 


ELECTRICAL ENGINEERING TESTING 


409 


This low voltage required at the normal frequency of the 
transformer, and from whatever source obtainable, must he 
adjusted by a suitable variable resistance, in series with P, so 
as to give eight or ten currents through P, varying by about 

equal amounts from its full-load value to the lowest readable, 

the readings of Vp, W, A and As being noted at each. 

(3) Disconnect the secondary short circuit and note the iron- 
loss reading, Wj on TF, for the saine value of Vp as used in obs. 2 
(if any is readable). 

(4) Measure the resistance rs of the short circuit (namely, As 
and its two short connecting leads) and tabulate your results as 
follows— 

Transformer: No. = . . . Made by . . . Type . . . Voltage Ratio = . . . 
Full load : Outpiit= KVA. Amps. = . . . Volts = . . . Frequency = . . . 

Resistances: Piiinary (Apl = . . • olitns @ . . . “(7 Secondary = . . . ohms 

@ ... °C. Secondary short-circuit (r^) = . . . ohms. 



(5) Plot a curve having values of copper loss Wg as ordinates, 
with A as abscissae. 

Note. —The impedance voltage Vp is entirely spent in over¬ 
coming the equivalent impedance of the windings with short- 
circuited secondary, being partly spent in overcoming resistance 
and partly in reactance. 

The reactance voltage = J Vp'^ — {WlA)\ 


























410 


ELECTRICAL ENGINEERING TESTING 


Deduction of the Regulation of a Trans¬ 
former for any Load and Power Factor 
from the ‘‘ Open ” and Short Circuit ” 
Tests. 

From the curves obtained in the preceding open and short- 
circuit tests, the drop in volts in a transformer on non-inductive 
or inductive secondary load can be predetermined. To obtain 
this drop is needed the “ open-circuit ” volts and the triangle of 
voltages relating impedance voltage, ohmic drop or resistance 
voltage, and the reactance voltage as obtained from the “short- 
circuit ” test. 

The voltage drop in the transformer for any load and power 
factor can thus be obtained from an exactly similar construction 
to that given on p. 182 for an alternator, and which will not, 
therefore, be repeated here. 


(138) Determination of the Regulation of 
a Static Transformer. (Differential 
Method.) 

Introduction. —The meaning of the term “regulation,” as 
applied to a transformer, was explained and defined in test 
No. 139, p. 412, and its measurement in a single transformer 
there given. When, however, two similar transformers 2\T^ 
are available, the present method of measurement is both simple, 
convenient, and direct reading, whereas that of test No. 139 
necessitates taking the difference between two voltages, and is 
less accurate. It is applicable to any pair of high-tension or 
low-tension transformers, but the secondary circuit should prefer¬ 
ably be the L.T. side, on account of the greater safety in handling 
instruments at low tension. 

Apparatus. —Source of A.C. supply E, whether low or high 
tension; two transformers similar in all respects; volt¬ 

meter Vs] switches SpSs] load resistance R] ammeter Ag' 
and (if available—for interest, but not as a necessity) two 
ammeters A pj and Ap.^. 


ELECTRICAL ENGINEERING TESTING 


411 


Observations. —(1) Connect up as in Fig. 160, levelling and 
adjusting to zero such instruments as need it. 

(2) First connect for Vs a voltmeter capable of reading (or 
glow lamps capable of absorbing) the sum of the normal voltages 

Then with Ss and Sp open, and E giving the normal 
voltage and frequency of or close Sp. If Vs shows a 
fairly large voltage, S-^^ and S^ are in helping series, and the 
connections of one of them must be interchanged to bring their 
voltages into opposing series, when Vs will show very little. 

(3) Now replace Vs by a low-reading voltmeter, and, with Sp 

closed [Ss still being open), note the readings of Ap^ and 
Fg (if any). If either exactly similar, or unloaded, or 

both, Fs should now read 0, 



(4) With the supply voltage constant, and R non-inductive and 
full in, close Ss, taking the readings of all the instruments for 
each of a series of six or eight load currents As, rising by about 
equal increments from 0 to the full-load current of or T^. 

Note.— Vs gives the difference of the voltages between the 
terminals of the loaded {T^ and unloaded transformer, 

which is the required “drop.” 

The secondary output of transformers is usually expressed in 
kilo-volt-amperes (K.Y. A.), irrespective of the power factor of the 
secondary circuit, and not in true K.W. at unity or some lower 
P.F. The values of Vs may, however, be obtained if desired on 
inductive loads by repeating obs. 4 with a variable choker {C) 
(not shown), connected in series with the non-inductive resist¬ 
ance R (preferably a bank- of lamps) and a voltmeter with key 
to measure the volts Vp across R and Vc across the choker at 
the same current, when the power factor of the circuit will be 

. Vn 

cos d = ^- = T7“- 

impedance Vo 
















412 


ELECTRICAL ENGINEERING TESTING 


Tabulate your results as shown— 


Currents for reference. 

Secondary 
Load Current 

Voltase Drop 

Vs- 

j4. T> * 

-p • 

2 






(5) Plot curves having values of Vs, as ordinates, with values 
of Js as abscissa©. 

(139) Determination of the Efficiency and 
Regulation of Transformers. (Single 
Conversion Method.) 

Introduction. —This method is one of the simplest, though not 
the most accurate, and entails using only the one transformer to 
be tested. In all cases by the primary of the transformer is 
meant that winding connected to the supply mains whether these 
are at high or low tension. 

The efficiency of any transformer, supplied at constant voltage 
and frequency, is the ratio of the secondary output to the 
primary input, or TTg/ 

The regulation of a transformer is the amount by which the 
secondary terminal voltage at any secondary load differs from 
that on open secondary circuit, i.e. it is the “ drop ” in voltage 
under load, and is due to both the resistance and reactance of 
the winding. 

The regulation curve therefore relates secondary terminal 
voltage as ordinates and secondary load current as abscissae. 
The ordinate intercept between this curve and a horizontal 
straight line through the “open circuit^’ voltage point at any 
secondary load gives the “drop” of voltage at that load. 

Caution. —In the case of transformers which have to be run 
off a high tension alternator and are tested by this method, the 
one operating the high tension instruments must not only wear a 
pair of carefully selected and good india-rubber gloves, but must 
also stand on an india-rubber mat, and to guard against the pos¬ 
sibility of accidents even in the case of manipulating the low 
tension instruments, the one operating these must either wear 
the pair of india-rubber gloves provided or stand on an india- 















ELECTRICAL ENOINEERING TESTING 


413 


rubber mat. Before switching on, the “ danger boards ” provided 
must be placed close to the high tension wires. 

Note. —Great care must be taken that the india-rubber gloves 
are not scratched, cut, or pierced in any way, as this would tend 
to render them useless for the purposes of insulation. 

Apparatus. —Alternating current ammeters (Fig. 251), 

and voltmeters non-inductive Wattmeters IFj Tfg (with 

their separate anti-inductive resistances r-^r^y if any); load 
absorbing resistance R, preferably non-inductive (p. 598); switches 
S 2 ; source of alternating current supply and transformer 
T to be tested. 

Note.—Fj and Fg should be either hot-wire or electrostatic 
.instruments, of which Fg may preferably be of the latter type. 
If R is strictly non-inductive, then TF 2 could be dispensed with; 
it may, however, as well be used if available. 



Fig. 161. 


Tests.—(1) Measure the ohmic resistances R^ of primary and 
R^ of secondary coils in a suitable manner. 

(2) Connect up the apparatus as indicated in Fig. 161, 
carefully levelling such instruments as need it, and seeing that 
their pointers are at o. Adjust the voltage and frequency (if 
possible) of the supply to the normal value required for the 
transformer. 

(3) With S 2 open, close and note the readings of Fj and 
IFj simultaneously. The “open circuit losses” occurring in the 
transformer will thus be obtained. 

(4) Make R large, and close as well as Sy Then note 
simultaneously the readings of all the instruments for about ten 
different secondary currents from 0 to full load or to 15% over 
full load, rising by about = increments. 



























414 


ELECTRICAL ENGINEERING TESTING 


In all cases the frequency and primary voltage must be kept 
constant. 

(5) Repeat 4 with a higher and lower frequency than the normal. 

(6) Repeat 4 and 5 on an inductive load of constant power 
factor, or otherwise obtain the readings, as detailed on p. 182, 
necessary for plotting the regulation curves between secondary 
volts and current, each at different but constant power factors. 

(7) Find, experimentally, the copper losses in primary and 
secondary by passing direct currents of various strengths, between 
0 and full load, through the coils, and noting the losses by means 


of a Wattmeter. 



Namb. . . 


Date . . . 

Transformer: No. . . . 

Made by . . . 

Transformation Ratio . . . 


Normal output = . . . Kilowatts. Frequency = . . . per Sec. Type . .. 
Primary Resistance R\ = . . . ohms at... °C. Secondary Resistance Rx = ... ohms at... °C. 



(8) Plot the following curves having values of— (a) Total 
copper losses; {h) total iron losses; (c) secondary voltage; 
[d) primary power factor; (e) efficiency; (/) voltage ratio, 
respectively as ordinates and secondary load currents as abscissae 
in each case. 

Inferences. —State concisely all the inferences which you can 
draw from the results of your experiments. 


(140) Determination of the Efficiency of Trans¬ 
formers. (Double Conversion Method.) 

Introduction. —This method can be use*d when two similar 
transformers are at hand, and particularly when only low tension 
measuring instruments are available. 

Caution. —To guard against the possibility of accidents, even 
in the case of manipulating the low tension instruments, the pair 




































ELECmiCAL ENGINEERING TESTING 


415 


of india-rubber gloves provided must be worn by the one manipu¬ 
lating the tertiary circuit instruments, and the india-rubber mat 
must be used by the one reading those on the primary circuit. 
On no account must any part of the secondary {Jiigh tension) circuit 
he touched while alivef and before switching on the primary 
current, the “danger boards” provided must be placed close to 
the high tension leads. 

Note.—Great care must be taken that the india-rubber gloves 
are not scratched, cut, or pierced in any way, as this would tend 
to render them useless for the purposes of insulation. 

Apparatus. —Alternating current ammeters A (Fig. 251), and 

voltmeters Fg, of which Fg is not absolutely essential to the 



Fra. Wz. 


test; non-inductive Wattmeters and Fg (with their separate 
anti-inductive resistances r^, if any) switches S-^ S 2 J load 
absorbing resistance R, preferably non-inductive (p. 598); source 
of alternating current supply and the two transformers 
to be tested. 

Note.—Both Fg and the high tension voltmeter Fg should, if 
possible, be of the electrostatic type. If R is strictly non- 
inductive, then Fg can be dispensed with it may, however, as 
well be used if available. 

Tests._(1) Measure the ohmic resistance of each of the coils 

of the transformers and in a suitable manner. 

(2) Connect up the above apparatus as indicated, carefully 
levelling such instruments as need it, and seeing that their 
pointers are at zero. Adjust the voltage and frequency (if 
possible) of the supply to the normal value required for the 

transformers. 





























416 


ELECTRICAL ENGINEERING TESTING 


(3) With >^2 and Sq open, close and note simultaneously the 
readings of A^, V^, Wy The “ open circuit losses ” occurring in 
transformer will thus be obtained. 

(4) Make R large and close all the switches. Then note sim¬ 
ultaneously the readings of all the instruments for about ten 
different tertiary currents from 0 to full load, rising by about = 
increments. 

(5) Interchange and so that the latter now becomes the 
“ step-up,'' and repeat exp. 3 and 4, tabulating your results 
in two tables similar to that shown. 

Note. —In all cases the frequency and secondary voltage must 
be kept constant. 

(5a) Hepeat 3—5 for a higher and lower frequency than the 
normal. 

(6) Find, experimentally, the copper losses in each of the coils 
by passing direct currents of various strengths between 0 and 
full load through them, and noting the losses by means of a 
Wattmeter. 


Name . . . 

Transformer Tj / No. . , . 

^2 I »» • • • 

,, f 


Date . . , 

Type . . . Made by . . . Used as . . . 

II * • • II • • • II • • 

sec. 


T2 \ Normal Output _ Kilowatts. Frequency _ 
Change ratio _ ' " 


Tertiary. 


Watts. 




0 

E-i 






Primary. 


Watts. 




cj 


o 

o 


11 

CO 

o 

O 


bO 

cd 

(M 

o 

'S) 




o . 

HR 

a I 

03 

oi R 
o 

3 i 

H^ 


Copper Losses 
in Watts. 


0^ 


C 4 


cc 


CO 

CO 

CO 

CO 

>3 ' 

o 

f-t 


Efficiency of 




0 

O 

cd 

fl 

•r^ 

.Q 

s 

o 


Vi 

03 

a 

C 

<21.« 
or? 

C 




Resistances: Primary = ... ohms. Total Secondary = ... ohms. Tertiary = ... ohms, at °C. 
Frequency used = ... per sec. 


Total Secondary drop = ... volts. 


2-2 


(7) Plot the following curves having values of— (a) Total 
copper losses; (b) total iron losses; (c) tertiary voltage ; (d) 
power factor ; (e) efficiency respectively as ordinates and tertiary 
load currents as abscissae in each case. 

Inferences.— State clearly all the inferences which you can 
draw from your experimental results. 














































ELECTRICAL ENGINEERhXG TESTING 


417 


(141) Efficiency of High Tension Trans¬ 
formers. (Sumpner’s Differential Method.) 

Introduction. —A neat and convenient method of measuring 
the efficiency of high tension transformers, and which is sus¬ 
ceptible of greater accuracy than most methods, is that due to 
Dr. W. E. Sumpner, and detailed as follows;—A small auxiliary 
transformer (C), the output of which need not be greater than 
the waste of power occurring in the two larger transformers 
A and B, to be tested, at full load, is required for the purpose of 
furnishing a small extra voltage (say 5 to 12 volts) necessary for 
driving the full load or any other currents through A and B. 
Its efficiency, goodness, or badness is a matter of indifference, 
and all we need in connection with it, is the output of its 
secondary in Watts as measured by the Wattmeter (ffii). 

The particulars as regards this output can be deduced as 
follows :—Suppose that two 2250 Watt transformers have to be 
tested each converting from 100 to 2000 volts or vice versd. 
Then their probable efficiency would be about 94% (say), and 
hence each would absorb 22*5 x6 = 135 Watts at full load. 
Consequently the output of the auxiliary transformer G need 
not exceed 2 x 135 or 270 Watts. 

Hence if used on low-pressure 100 volt mains the primary 
should take about 2*7 amps, at 100 volts, and the secondary give 
out 22*5 amps, at 12 volts. In the ordinary test of efficiency of 
high tension transformers, in which two similar ones are used— 
one as a step-up from the low-pressure primary mains, and 
the other as a step-down to the tertiary mains, the efficiency is 
deduced from measurements of primary imput and tertiary out¬ 
put which are of nearly equal magnitude. Consequently the 
percentage error in measuring these two quantities re-appears 
as the same percentage error in the efficiency so obtained. 

The present method, which is much to be preferred of the two, 
consists in actually measuring the losses (to) occurring in the two 
transformers directly, and comparing these with the input to 
obtain the efficiency. 

The method is economical in cost of energy used, especially 
when testing large transformers; with the methods used in tests 


418 


ELECTRICAL ENGINEERING TESTING 


139 and 140 it would be a serious consideration, while the 
supply of full-load current would make a serious demand on a 
public supply, or necessitate a large testing alternator. The 
present method is accurate because the total loss (w) in the two 
transformers is obtained by adding together two quantities, and 
not by subtracting them, and is most convenient for finding the 
temperature rise after a run of a prescribed number of hours on 
full load. 

The principle of the present method is strikingly analogous to 
Dr. Hopkinson’s combined efficiency test of a pair of dynamos, 
the distinguishing feature of which is to couple two similar 
machines together both mechanically and electrically, one to 
run as a dynamo and the other as a motor. Energy is supplied 
to one by which it is transferred to the other, this latter return¬ 
ing it again to the source; the balance of energy supplied actually 
by the source is therefore equal to the waste which occurs in 
the double transformation and corresponds with the loss {w) 
above mentioned. This then is what takes place in the present 
case, for energy is taken from the mains by the “ step-up 
(A or E, whichever is used as such), then transferred to the 
‘^step-down’’ transformer, and finally back to the mains again. 

Thus, while both transformers can be loaded to any extent by 
controlling the current circulating between them, the power 
taken from the supply is only some 4 to 20 % of the full-load 
K. W. capacity of either, depending on their efficiency—being only 
that necessary to make up the total internal losses in the two 
transformers together. AVhether the L.T. or H.T. windings are 
connected to the supply is merely a matter of convenience de¬ 
pending on which supply is available, but usually the L.T. sides 
are connected to an L.T. supply for safety in handling the more 
commonly available low-tension instruments, etc. Calling which¬ 
ever are connected to the supply the primaries, the secondaries 
must be so connected in series that their E.M.F.s oppose each 
other. If the primary E.M.F.s are equal, so also will be the 
secondary E.M.F.s, and no current will flow in the secondary 
windings. By making the small auxiliary transformer in series 
with one of the primaries provide a or —boosting E.M.F., 
the out-of balance primary E.M.F.s so produced will cause 
out-of-balance secondary E.M.F.s and a circulating secondary 


ELECTBIGAL ENGINEEEING TESTING 


419 


current, the strength of this current depending on the difference 
between the E.M.F.s. Should the connections be such that the 
secondary E.M.F.s are in helping series instead of opposing 
series, as they should be, a short circuit will result. To avoid 
this, and to ensure the connections of the secondaries being 
correct, close S and S^, when B will induce a voltage in the 
L.T. winding of A equal to that of the supply, but opposite in 
phase if the connections are correct. Hence, if either a volt¬ 
meter or lamps, each having a voltage range equalling twice 
that of the supply, are connected across the open switch and 
neither show any voltage, the connections are correct for the 
two L.T. windings, and therefore also the two H.T. windings 
are then in opposition. If otherwise, the voltmeter or lamps 
will show tioice the voltage of the supply. In this event the 
connections of one of the H.T. secondaries must be reversed. 
It should be noted that {a) will indicate the load current, while 
an ammeter (^j) in series with will give the magnetizing 
current. 

If W= load in Watts supplied to the primary of the “ step-up,” 
and ^c^^C 2 = the Watts at this load as measured by w-^ and 
and A. = loss of power in the connecting leads, current meter a 
and the current coil of TTi, then the total loss in the two 
transformers 

tu = ICj -f 1^2 - A. 

w 

Hence the efficiency of double conversion = \ 
and the efficiency of either transformer = 



As the ratio of ^ is small—not greater than with a trans¬ 
former of 95% efficiency, the efficiency of each transformer is 
given quite accurately enough by the relation 


2 TIA 8 jp 


An error of 10% in estimating -p^only affects the combined effici¬ 
ency to 1%, and that of either transformers to J% only. Hence 
can be seen the superiority of the present method over the 
preceding one. The quantity W can be obtained with quite 



420 


ELECTRICAL ENGINEERING TESTING 


sufficient accuracy by the product of the current A and the P.D. 
in volts supplied to the primary of the “step-up.’^ 

If (^) is the “step-up,” then 1F= 100 x current in low tension 
coil of A, whereas if R is the “ step-up,” the power returned to 
the mains by ^ = the above quantity, and hence the input of 
the primary of B is 

jP'= 100 X current of -4 + (w^ -f 

That transformer will be acting as “step-up” which has the 
higher P.D. of the two {A and B) on its low tension coil. If, 
say, 12 volts are supplied by G to the primary of B, the P.D. at 
the terminals of B will be either 112 or 88 volts according as 
the 12 volts from the auxiliary and the 100 of the mains are 
in phase or opposite phase. If R was very inductive, the above 
voltages would be out of phase, and B's P.D. might be anything 
between 88 and 112 volts. 

Apparatus. —The two high-tension transformers A and B to 
be tested of say 2000/100 volts; an auxiliary Boosting one (7, 
the primary of which is in series with a variable non-inductive 
resistance R of sufficient range to produce only a few secondary 
volts; two non-inductive Wattmeters TFi TFg i Siemens electro¬ 
dynamometer or direct reading alternating current ammeter [a ); 
switches and S ; voltmeter V for maintaining the mains 

at 100 volts; alternator D, or some other source of alternating 
current. 

Caution. —On no account whatever is the high tension circuit 
of either A or B to be touched while “ alive.” The india-rubber 
gloves and mat must be used by the operators to ensure immunity 
from accidental shock, or break-down of the insulation between 
primary and secondary of A and B, 

Observations. —(1) Connect up as in Fig. 163, and adjust the 
instruments to zero where necessary. See that all lubricating 
arrangements are working properly, also that the gloves are in 
good order and the mat suitably placed. 

(2) Measure the losses due to the resistance of the leads and 
instruments employing alternating currents, which can be done 
without altering any of the connections thus—with E and 
open, short circuit the primaries of A and B and close S 2 

(3) Vary R so as to obtain about six different currents through 
{a) from 0 to the full load current of .4 or 15 by causing the 
secondary voltage of C to vary suitably. Note the corresponding 


ELEGTEIGAL ENGINEERING TESTING 


421 


readings on JV^ (the power given out by the secondary of C), which 
therefore at once gives the required losses in Watts in leads 
and instruments for each particfular current passing through them. 

(4) Measure the copper losses in the coils of the two trans¬ 
formers A and B by opening and closing 
observing the readings of W-^ for some six different currents from 
0 to the maximum of A and B, as read off on a. 


(5) Measure the iron or core losses in the two transformers A 
and B by closing all the switches and observing the reading of TFg. 



(6) With and open, close and 8 and note the readings 
of ^2 ^^ 2 * Then will indicate twice the no-load 

current of either transformers A or i?, and JFg twice the no-load 
losses in either. 

Tabulate your results as follows— 

Name . . , Date . . . 

Transformer used as Step-up: No. ... Type... Make...: Resist. Rp = ... Rg = ... Olims. 
,, „ Step-down: No.... „ ... ,, ...: „ Rp = ...Rg = .., „ 

„ No.... Output: Volts = ... Amps. = ... Constant of Wattmeter JFi =... „ 


Si>eed in Revs, 
per min. JV. 

) 

( 

•=ofe;(o 

C 11 

aj (U 11 

S P4 si 

a< ^ 

<a 

*0 

Current. 

Total Losses in 

Total Input into 
Primary of Step-up 

IF = 100 a + w. 

Efficiency 

Reading on 
a. 

Amps. a. 

Coils, Leads and 
Instruments 
(JVi in 4) wi. 

S I.®" 
itib 

Leads and 
Instruments 

Wi in 3=A,. 

r» ^ 

^ ^ 

§ 

of Combination 

in% 

100 

of either 
transformer 











































































422 


ELECTRICAL ENGINEERING TESTING 


(7) Plot the following curves on the same curve sheet having 
currents (a) as abscissa), and for the ordinates the following— 
(i) Losses in leads and instruments; (ii) iron core losses; (iii) 
a^R losses in the coils of A and B) (iv) total a^R losses in 
A and B. Also with efficiency as ordinates, and load W in 
AVatts as abscissae. 

(8) Reverse the positions of A and B, and repeat the above tests. 

Inferences. —State very clearly all that you can infer from 

your experimental results. 

(142) Measurement of the Efficiency of ordi¬ 
nary Single-Phase Transformers by 
Blakesley’s 3-dynamometer Method. 

Introduction. —This method necessitates the use of two 
ordinary Siemens electro-dynamometers, in which of course the 
moving coil is in series with and carries the same current as the 
fixed coil, whence the angle of torsion is proportional to the 
\/mean square value of the alternating current, and in addition 
the use of a third Siemens electro-dynamometer, arranged so 
that the moving coil has its own separate terminals, and is not in 
series with the fixed coil. 

If then two alternating currents of equal period, from either 
the same or different sources, flow through the two independent 
coils, the periodic time of oscillation of the moving coil being 
very large compared with that of the current, the angle of torsion 
is proportional to the mean product of the simultaneous instan¬ 
taneous values of current throughout the period, and is called the 
“ split dynamometer ” reading. 

If Aq and ^^1 = maximum values, and A, A^ the mean values of 
two simple periodic alternating currents, one of which lags behind 
the other by an angle a, then the ordinary dynamometer will 
give A = ^Ao^ and A^ == ^{A^y. On passing these currents through 
the split dynamometer its reading ^ would he=> ^A^Aq^ cos. a, and 

hence cos. a = 

The follov*ing method is quite general, and does not assume 
that the current is a simple sine function of the time, but does 


_ ^ 

^/aa^' 





ELECTRICAL ENGINEERING TESTING 


423 


assume that there is no magnetic leakage, i. e. that the number of 
lines cutting the primary and secondary are the same. This is 
not true in all types of transformers on full load, but is nearly so 
in closed magnetic circuit types. 

Since the split dynamometer gives no reading on open 
secondary circuit, this method is useless for determining the 
“ open circuit ” losses. 

Apparatus. — Two ordinary Siemens electro-dynamometers 
and one split dynamometer (^A) ; transformer T to be 
tested; non-inductive resistance L (such as a bank of lamps to 
take up the secondary load) (p. 598); alternator D ; switches 
voltmeters ; non-inductive Wattmeter W, inserted merely 

for the purposes of comparison. 

Observations. —(1) Connect up as in Fig. 164, and adjust 
the instruments to zero where necessary. See that all lubricating 
. cups in use feed slowly and properly, then start I). 

(2) being open, close and adjust the speed and excitation 



Fig. 164. 

so that Fj reads the normal voltage required for the primary at 
the normal frequency of the transformer. Note the readings of 
Jj, Fj and IF. 

(3) Close S .2 and adjust L so that A^ reads about of the 
maximum secondary current, the voltage Fj being kept normal by 
varying the excitation. Now note the readings of A^ Aj, A 2 , 

and IF. 

(4) Repeat 3 for about 10 secondary load currents, rising by 



































424 


ELECTRICAL ENGINEERING TESTING 


about equal increments to the maximum allowable, and tabulate 
as follows— 


Name. . . 


Date . . . 


Transformer tested : No. , 
Primary turns Ni = . . . 
Secondary „ Ng = . . . 
Normal: Volts . . . 


. Type . . . Make . . . 

Resistance iJi = . . . ohms, at . . . *G. 

,, • >> . . . C. 

Amps. . . . Transformation Ratio = . . . 


U 

o 

P4 


V. s 

(U 

p< 

m 


h 

<0 

u 


Dynamo¬ 

meter 

Reading. 






Amps. 




>> 

P-i 

■S w 

0 +3 

o 

Sh cS 
c3 

Ph 

< 


+» 

P 

^ If 

»-l II 
^ ft 

c3^ 

5 

dl 




tei 


C3 

Pi . 

. O 

E lx 
O A* 

k II 

'd 

o 

a> 

00 


to 

<u 


(N 

rH 

to 

rt 

dl 




o 

cS 

O) 

o 


>> 

u 

o 

o 

59 

H 


(5) Plot curves having values of TT^as abscissae, with efficiencies 
and Fg ordinates. 


(143) Measurement of the Efficiency of Multi¬ 
phase Alternating Current Transformers. 

Introduction. —The determination of the efficiency of ordinary 
single-phase transformers has already been fully considered in the 
preceding pages. 

The present test does not differ materially in principle from 
those in question, and practically the only difference is in 
the method of measuring the power absorbed and developed by 
the’ multiphase transformer, and which possesses some character¬ 
istic differences from that used in the case of the ordinary single¬ 
phase transformer. 

Most of the preceding methods are equally applicable in the 
present case whether the transformer is of the two or the three 
phase type. The reader should refer to p. 388 for the method of 
measuring electrical power in two and three phase alternating 
current circuits, where a more detailed description of them will 
be found. If in the present instance, as in fact with any others, 
the rheostats or circuits in which the load is to be absorbed are 
strictly non-inductive, i. e. are of the nature of incandescent lamps 
or water rheostats, then providing such load-absorbing devices 
operate equally on each of the sections of the circuit, thus main- 










































ELECTRICAL EXGINEERING TESTING 


425 


taining a balanced system, the output can quite accurately 
enough be obtained from the ammeter and voltmeter readings in 
the manner set forth on pp. 388 et seq. 

For the present test we will assume that the efficiency of a 
3-phase transformer is required by, say, the single conversion 
method. 

Apparatus. —The 3-phase transformer to be tested, of which P 
is the primary winding and E the secondary shown in Fig. 165, 
with the star or open winding; two non-inductive Wattmeters 
and TFg; three Parr’s direct-reading dynamometer ammeters 
Ay Aj^ and A^ (p. 572); three voltmeters V, Fj and F 2 ; 3-phase 
variable rheostat R (non-inductive) capable of operating equally 
on each line (p. 6 O 8 ); source of 3-pbase current E ; two 3-throw 
switches ESS and E-^ E^ E^. 

Note.—If the 3-phase rheostat E is not non-inductive, then 
two additional Wattmeters will be necessary in the secondary 



circuits connected up in precisely the same way as those shown in 
the primary circuit, the secondary output being then given by the 
sum of their readings at any particular load. 

Observations.—(1) Connect up as in Fig. 165, and adjust all 
the instruments to zero, levelling such as require it. See that 
all lubricating cups in use feed slowly and properly if the source 
of 3-phase current supply E is controllable. 

(2) With aS'j S^ S-^ open, close ESS, and adjust the speed of the 
generator so as to give the proper periodicity for the transformer 


































426 


ELECTRICAL ENGINEERING TESTING 


and then the excitation, so as to hav^ the desired voltage, shown 
by V across the primary. 

Note the respective Wattmeter readings TFj and 11^2) 
possible that of A in addition to V. Then (ir^ + IFg) = the no- 
load jn'imary input — the magnetizing losses. 

(3) With R at its maximum, close and note the readings 

of all the instruments for some ten or twelve secondary load- 
currents from the smallest to the maximum permissible, rising 
by about equal increments at a time for constant secondary 
voltage. 

(4) Calculate the secondary loads (IF^) from the relation— 

1 ^ 5 = A-^V-^= A^V^^^ etc., 

and tabulate as follows— 

Name . . . Date , , . 

Transformer tested: No. . . . Type . . . Maker . . . 

Normal: Volts . .. Amps. ... Periodicity . .. AVatts . . . Constant of \ IFi = . . . 

Resistance : Each Primary coil Ri — . . . ohms. @ . . . * C. AVattmeter. / JFj = . . . 

„ : „ Seconding coil R^ =.. . ,, @ . . . ” C. 


Primary Circuit. 

u 

o • 

O ^ 

Angle of Lag 

Secondary Circuit. 

Total Loss in 

O » 1 (i, 

P liw 
— IS 

‘3 o 

js s 

« V 

R- 

Volts V. 

Amps. A. 

Apparent Watts 
^J^AV. 

1 + 

a> ^ 

5 11 

U 

o 

tr 

o 

> 

A 

u 

o 

a 

p 

. 

r4 

a ^\co 

Transformer 

1 Wp - 

Copper Coils 
Wc- 

t) 

o 

^ 1 

O ' 

2 1 















(5) Measure the resistance of the transformer coils by means 
of either the Wheatstone Bridge or Potential Difference method. 
Plot the folloAving curA’^es betAveen— 

EflSciencies r) as ordinates 
abscissae. 

Power Factor as ordinates and secondary loads as 

abscissae. 

Inferences.—State clearly all that can be inferred from your 
experimental results. 


and secondary loads (\/3JjFj)as 


































ELECTRICAL ENGINEERING TESTING 


427 


(144) Efficiency of a Nodon Valve Electro¬ 
lytic Rectifier. 

Introduction. —The necessity of obtaining continuous current 
for certain purposes, such as electrolytic work and the charging 
of secondary cells, where, frequently, the only available public 
supply is alternating current, has led to the introduction of 
rectifiers for rectifying alternating into continuous or unidirec¬ 
tional current. 

Of such appliances, there are now several commercially 
successful forms; that known as the nodon valve consists of 
as many pairs of cells grouped according to the Leo Gratz 
method (Fig. 166) as there are phases of current or distributing 
mains, in order to obtain a single rectified current. Each cell 
consists of plates formed of an alloy, mainly composed of 
aluminium, acting as cathode, immersed in a solution of borate 
or phosphate of ammonium or other salt formed from tartaric, 
acetic, oxalic or gallic acids. The solution is capable of rapidly 
altering the condition of the polarizing film formed by an 
alternating current on the aluminium. The containing cell is 
made of lead and constitutes the anode. The electrolytic action 
taking place is as follows— 

In one half period of the alternating current, current tends to 
flow from aluminium to lead, but cannot, owing to an insulating 
film of very high resistances being formed over the aluminium 
(cathode) plate. In the next half (reversed) period, the current 
actually is able to flow from lead to aluminium owing to the 
instantaneous de-polarization or reduction of the film on the 
aluminium plate. The principle on which both semi-waves of 
the period of an a.c. supply are utilized, is that proposed by 
Leo Gratz, and shown in Fig. 166, for a single phase alternating — 
to direct—current transformation. A and L are the aluminium 
and lead plates respectively of the four cells I, /, and //, II. 
The continuous arrows represent the direction of current in 
the valve during one half of a period when the current of the 
alternating supply flows from P to R. The dotted arrows show 
the direction of current in the valve in the next half period 
when the supply current flows in the reverse direction from 


428 


ELEGTRIGAL ENGINEERING TESTING 


R to P, Thus for the first half period it is blocked in cells //, 
II, and in the second half period it is blocked in cells I, I. 
A unidirectional current therefore always flows from D to G 
through any load (?•) whether motor, secondary cells or resistance, 
etc. To obtain greater constancy or uniformity of d.c. voltage 
a suitable condenser can be connected across D and C. A single 
valve will stand a.c. pressures up to 140 volts between Q and 
R, that between D and G being about 90% of this. For higher 
a.c. pressures two or more valves may be combined, or an 
“economy coil” type of transformer connected between valve 
and a.c. supply. The pressure between D and G may be 
varied to any extent by a corresponding variation of that of the 
supply between Q and R. 

The temperature of the electrolyte must not be allowed to rise 
much above 50° G., and in large valves, forced air draught around 
the cells is resorted to in order to keep down the temperature. 
The valve may be used on any periodicity employed in practice 
up to 100 per sec. or more. A starting resistance {S) must 
be employed with the valve when this has been out of use for 
a few hours, in order to reform the insulating pellicule on the 
aluminium plate. This only takes a few seconds and prevents 
a sudden heavy rush of current through the valve. The resist¬ 
ance or inductance of S is cut out entirely afterwards. 

Evaporation of the solution is made up by adding distilled 
water and the solution need only be renewed at long intervals. 

Apparatus. —The nodon valve complete; starting resistance 
{S) j alternating current ammeter (^), voltmeter (F), wattmeter 
(IF) ; direct current ammeter (a), voltmeter {v ); load or variable 
resistance (r); thermometer; source of a.c. supply; economy coil 
or transformer if a.c. supply exceeds 140 volts; switches S-^, 

Observations. —(1) Connect upas in Fig. 166, and adjust all 
the instruments to zero, levelling such as require it. Q and R are 
the terminals marked ALT on the valve, and are to be connected 
to the a.c. supply: D and G are the terminals marked -p and —. 
If machinery is being run for supplying the valve, see that all 
oil cups feed very slowly and properly. 

(2) With and 8^2 ^ adjust the a.c. supply 

so that V reads about 140 volts, the periodicity being kept 
constant at normal value. Now close S-^, and note the readings 
of all the instruments. 


ELECTRICAL ENGINEERING TESTING 


429 


(3) With S 2 still open, gradually cut out S to short circuit and 
again note all instrumental readings and the temperature of the 
electrolyte. 

(4) Re insert S and with (r) full in, close /S'g and gradually 
cut out {S) to short circuit. Next adjust (r) so that (a) reads 
about 3 ^th full output current and note the readings of all the 
instruments. 



(5) Re-adjust (r) so as to obtain some ten different load 
currents on (a) rising by about equal increments to the maximum 
for which the value is intended, and note the temperature and 
readings of all instruments at each. 

(6) Repeat (5) for a widely different but constant periodicity 
(if available) above and below normal at the same voltage if 
possible. 





















430 


ELECTRICAL ENGINEERING TESTING 


(7) Repecat (5) for a constant supply voltage, say 50% less 
than before, at normal periodicity. 

(8) Open {S 2 ) and at constant normal periodicity, note the 
readings of all the instruments for ten different voltages between 
0 and 140 volts. 

(9) With a convenient constant supply voltage and /S'g open, 
take readings of all the instruments for ten different periodicities, 
ranging from the maximum obtainable downwards. 

(10) Repeat both (8 and 9) for S 2 closed, constant full load 
being maintained on (a) by varying (r), and tabulate all results 
as follows— 

Name . . . Date . . . 

Nod on Valve: No. . . . No. of cells . . . 

Area of Anode ... sq. in. Area of Cathode . . . sq. In. 

Maximum output Amperes = . . . 


Value 

of 

S. 

Tempr. 

of 

Solu¬ 

tion. 

Periods 

per 

Sec. 

. Primary. 

Secondary. 

Voltage 

Ratio 

of 

conver. 

sion 

V 

T 

Effici¬ 

ency 

av 

W 

Volts 

in 

Amps. 

(A). 

Watt¬ 

meter 

Read¬ 

ing 

DIF. 

True 

Watts 

W. 

Ap. 

parent 

Watts 

AV. 

Power 

Factor 

W 

AV 

Volts 

(v). 

Amps. 

(a). 

Watts 

iav). 










1 






Note.—If the valve is cooled by forced air draught, the power 
absorbed in producing the draught must be added to the true 
watts (TF), or watts {av), according to whether it is supplied by 
the primary or secondary circuit respectively. 

(11) Plot curves on the same sheet, having values of—power 
factor; volts [v); efficiency; and voltage ratio as ordinates, with 
secondary load (av) as abscissie; ‘also between efficiency as 
ordinates and temperature as abscissie at constant secondary 
load. 

(12) Plot curves (for Exp. 8 and 10) with voltage as abscissie 
and the other quantities as ordinates; also (for Exp. 9 and 10) 
with periodicity as abscissae and the other quantities as ordinates. 

Inferences. —State clearly all the inferences deducible from 
experimental results. 




































ELECTRICAL ENGINEERING TESTING 


431 


(145) Efficiency of a Rotary Rectifier. 

Introduction. —Kotary rectifiers are employed for the purpose 
of rectifying single-phase alternating current into unidirectional 
or continuous current, and comprise a special form of commutator 
driven at synchronous speed by a suitable single-phase synchronous 
a.c. motor. 

The now well-known Ferranti rotary rectifier comprises, in 
addition, a constant-current static transformer, which, when 
supplied with varying a.c. at constant pressure, automatically 
delivers constant direct current at varying pressure for supplying 
arc lamps in series. Since the motor is driven in this case from 
a separate secondary coil on the static transformer, the ratio of 
the d.c. power output to the a.c. power intake by the primaries 
of the transformer gives the overall net efficiency which may be 
over 91% at a full load of 40 H.P. with a power factor of 0*90. 

In the Morton and Wright rotary rectifier there is merely the 
special commutator and synchronous motor, the rectification 
being from varying current at constant a.c. voltage to varying 
current at constant voltage on the d.c. side. 

Apparatus. —The rectifier complete comprising—motor J/, 
commutator JB, diphaser F and starting switch S; ammeters A,^, 
Aj^ and Aj) ; voltmeters V and Vu; switches Sj_ and Sj), 

and two-way voltmeter key K, wattmeter JV ; load absorbing 
device Rjy; and non-inductive regulating resistance (if 
necessary). 

Observations.—(1) Connect up as in Fig. 167. The terminals 
marked (m) Fig. 167, are for the motor circuit, and those marked 
DC and AC are the terminals for the direct and alternating 
currents sides of the rectifying commutator B. Adjust all the 
instruments to zero, levelling such as require it, and see that the 
bearings of the rectifier, and those of any other machine in use, 
are properly lubricated before starting. See also that the two 
brushes, which rub on the central-sectioned part of the rectify¬ 
ing commutator, are adjusted to touch on the thin strips 
simultaneously. 


432 


ELECTRICAL ENGINEERING TESTING 


(2) With the switch (^S) on the stud marked Starts close S-^ and 
Sm only, and move the brush rocker on the motor itself, until 
the machine emits a constant hum and runs quite sparklessly, 
when it will then be in synchronism with the supply. Now 
switch S to the right-hand contact and if necessary re adjust the 
position of the rocker to obtain sparkless commutation. 

(3) With K on note the readings of IT, and V. 



(4) With Rji full in and open close and again note W 
A^, V, Aa and Vj,, K now being on stud Va and in quick 
succession. 

(5) Close Sji and note the readings of all the instruments for 
about ten different currents on from 0 to full load, by varying 
Rd'j adjusting the brush rocker of the rectifying commutator to 
get sparkless rectification at all loads. 

Tabulate all your results as follows—> 

































ELEGTRIGAL ENGINEERING TESTING 


433 


Name . . . Date . 

Rectifier: No. . . . Type . . . 

Full-load Output =. Watts. Volts. 

Value of non-inductive resistance rm—... ohms. 


Maker . . . 
. . Amps. 


Wattmeter 
Reading Dw. 

True Watts W. 

Motor 

Volts Vj. 

CO 

P. 

s 

<1 

• 

1 

• 

e 

bC 

E 

b -f 

Q 

cn 

Amps. Ad- 

d 

QQ 

Overall Efficiency 

AdVij 

W-Am^m 

Voltage Ratio ot 

Vn 

Conversion —=L- 

Va 

Amps. Am- 

0 

> 

Apparent 

u 

0 

c3 ff . S 

kT ^ 
s 

0 












1 





(6) Plot the following curves :—between output Wj) as abscissae 

W- W„, 

and volts Vj); watts W; — . y- --; efficiency; and voltage ratio 

of conversion, as ordinates in each case on the same curve sheet. 

Inferences. —State concisely all the inferences which may be 
deduced from the results of the above tests. 


(146) Efficiency and Characteristic of Alter¬ 
nating Current Rotatory Converters. 
(Run from the Direct Current Side.) 

Introduction.—The rapid development of multiphase alternating 
current machinery, but perhaps more especially of that particular 
class of the same, known now commonly by the name of the 
Rotatory Converter, marks one important epoch in the history 
of this all-important and ever-increasing branch of industry— 
Electrical Engineering. There are several different types of 
transformers, but all come under one or other of two main heads. 

(1) Static transformers or converters with no moving parts. 

(2) Rotatory transformers or converters having moving parts, 
and on which latter their very existence depends. The former of 
course include the ordinary every-day transformer which we are 
so accustomed to see. 

The type 1 transforms electrical energy of one species at a 
particular pressure into the same species but at a different 
pressure, while type 2 transforms electrical energy of one species 
















































434 


ELECTlilGAL ENGINEERING TESTING 


into that of another. To this class belong the various forms of 
multiphase rotatory converters. Those converting from multi¬ 
phase alternating currents to continuous currents or vice versdj 
are usually multipolar machines, having any number of pairs of 
poles up to about 16 or more, with a periodicity ranging from 20 
to something like 60 per sec. Owing, however, to the con¬ 
ditions imposed by the relations between voltage, speed and 
size, they usually operate best at the lower periodicities. 

The rotatory converter to be tested consists of an ordinary 
direct-current machine, with the usual armature winding and its 
commutator at one end and three slip rings at the other, connected 
to three points on the armature winding—0, f and ^ of the polar 
pitch apart, i.e. in a two-pole machine at 120° apart. The 
machine when driven as a motor by direct currents taken in at 
the ordinary commutator end develops a 3-phase alternating 
current at the slip rings. It is this type of converter which is 
beginning to be used now on a large scale, only with more than 
one pair of poles, in long distance transmission of power, as 
follows—Polyphase alternating currents being transmitted at 
high pressure from the distant generating station, are reduced to, 
say, 100 to 300 volts by static transformers at the near end and 
then converted by the rotatory converter into direct currents, 
which may be employed for tramway, lighting, electrolytic pur¬ 
poses, or for charging storage cells. In any converting appliance, 
and therefore in any converter, the {total energy put in) - {total 
energy given out) {total internal losses). These are made up of 
mechanical frictions at journals, brushes, and due to wind or 
air churning, magnetic hysteresis, eddy currents and copper 
losses. 

Owing to the armature reactions of the dynamo and motor 
currents practically balancing one another, no lead of the 
brushes in either direction is needed for sparkless running. 

A rotary converter is usually run from the A.C. side in 
practice, but when “inverted,” i.e. run from the D.C. side, as 
in the present instance, the nature of the external circuit will 
have the same eflfect on its field as it has on that of an A.C. 
generator, but with additional effects. 

Thus on inductive load or power factor less than unity, a 
leading current will cause an armature reaction which will 
strengthen the field and hence reduce the speed, while a lagging 


ELECTRICAL ENGINEERING TESTING 


435 


current will cause a reaction that will weaken the field and 
hence increase the speed, in either cause producing a change 
of frequency. 

In fact, the increase of speed may become excessive from 
either small lagging power factors or short circuit causing 
excessive weakening of the field, which can only be counter¬ 
acted by an increase in the natural excitation of the field 
proportional to the effect causing the increase of speed. 

Apparatus.—Multiphase converter C to be tested in the present 
case assumed to be for 3-phase currents ; source of direct-current 
supply E ] direct-current ammeters A and a and voltmeter V ; 
alternating current ammeters A-^A^A^^ and voltmeters Fj Fg Fg ; 
non-inductive 3-phase rheostat 7?2 ordinary ones R 



(p. 606) and r (p. 599); triple-pole switch and S; non- 

inductive Wattmeters TFj and IF 2 ; tachometer. 

Xote.—Certain pieces of the above apparatus are not absolutely 
necessary to the test, but when available may preferably be 
inserted and used so as to clearly show what is actually taking 
place. Thus if the three resistances 7?^ R^ R^ constitute a proper 
3-phase rheostat (water or otherwise) (p. 607), which oioerates 
equally on each of the mains, then in addition to apparatus as 
before we need only have—one single 3-phase rheostat R-^^ R^ R^ ] 
one ammeter A-^ and voltmeter Fj instead of the three j one 
3-throw switch S 2 ^ 3 > close the three circuits simultaneously, 
and one Wattmeter. 













































































438 


ELECTRICAL ENGINEERING TESTING 


The reason for such an alteration is fully described on p. 389, 
in connection with power measurements in multiphase circuits 
that are symmetrically loaded. The method or rule for deducing 
the power absorbed in such cases will be found there, and 
must then be used. In the present case we will assume that the 
circuits are not symmetrically loaded. 

For a more detailed description of power measurements in 
multiphase circuits, see p. 388 et seq. 

Observations. —(1) Connect up as in Fig. 168, and adjust all 
the instruments to zero, levelling such as require it. See that all 
lubricating arrangements in use act properly. Increase ^^3 

to a maximum and r to a minimum. See that all the switches 
are open and that the brushes are fixed in the neutral position. 

(2) Start C like an ordinary D.C. motor, the speed and volts 

V being adjusted to, and kept constant at, the normal values. 
Take readings of all the instruments with open, and 

again with S-^ closed, for about ten different load currents 

on A-^ A^ A^ rising by about equal amounts to the maximum 
permissible by varying E^ E^. 

(3) The excitation [a) and volts (F) being now kept constant 
at normal value, repeat the readings in 2. 

(4) The speed and excitation being next kept constant at 
normal value, repeat 2. 


Namk . . . Date . . 

Rotatory Converter: No. . . . Typo . . . Maker . . . 

,, „ ; Normal output; Volts = . . . Amps. = . . . Speed = . . 

,, „ : Resistances ; Shunt Coils r, = . . . ohms at ... *0. 

Armature ra = . . . 

Normal ratio of conversion = . . . 

Periods per revol“. K= . . , 


O 

P? 

o 

oo 


o 

o 

09 

09 

Ck 


) 

I o 


>» 

u 

0 


Volts. 




Amps. 


(N 


CO 


Tnie 

Watts. 


Total 

H.P. 


.s 

p- 


•.S’ 

fa; 

+3 

0 

o 

0 

<D 

> 

fcO 


Total 

Losses 




O) 

.4^ 

P-( 

a> 

> 

P 

o 

O 


o 

6 

a 

a 

Q 

d 

I 

« 

m 

cO 

'd 

09 

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d 


O 

o 


c5 

P? 

P 

O 

•r^ 

09 

09 

► 

P 

o 

O 


09 

I 


CJ 


o 

o 


II 


o 
fl 

OJ /I 

o 

e 


tc 

V_A 

A- T 

St 

I- 

a> 

o 

Oh 



























































ELECTRICAL ENGINEERING TESTING 


437 


(5) Repeat 2-4 for a highly inductive circuit B^ B^. 

(6) From observations 2-5 plot the efficiency curve having 
as abscissae and 2 as ordinates. 

The external or a-c characteristic with as ordinates and 
as abscissae. 

The current characteristic with A as ordinates and A^ as 
abscissae. 

The voltage ratio, input, and speed as ordinates and ll^ as 
abscissae. 

From the current characteristic indicate how the efficiency of 
the converter could be calculated at any load and also work out 
the ratio of current transformation. 

Inferences.— State very clearly all that can be deduced from 
your experimental results. 

Note. —If 71 = number of armature windings per radian, 
e = maximum E.M.F. per turn of winding, 
and if we assume the flux in the interpolar space to be sinusoid¬ 
ally distributed, and that the E.M.F. is a sine function of the 
time, then the voltage ratio of conversion with 3-phase 
connections 


VI 

2 ne 



and the virtual voltage across any pair of slip collector rings will 
= 61*23 when that impressed on the direct current side = 100 
volts. 

In other words, the voltage ratio of conversion with sine 
distribution of flux in the interpolar space 

= 61*23%. 

As, however, the flux is never so distributed, and, moreover, as 
the voltage ratio depends to a large extent on the polar arc, pole, 
shape and position of the brushes, the above ratio is only roughly 
about what may be expected. 

The C^B total loss and temperature rise will be less in the 
machine used as a converter than when used as a dynamo. 



438 


ELECTRICAL ENGINEERING TESTING 


(147) No-Load (open circuit) Characteristic 
or Magnetization Curve of Continuous- 
Alternating Current Rotary Converters. 
(Run from the Continuous Current Side.) 

The No-Load Characteristic or curve of magnetization of a 
converter from which its magnetic properties and most suitable 
excitation is seen can be obtained in one of the two following 
ways— 

(1) By driving the rotary at constant speed from a direct 
coupled motor, or by belting, and noting the readings of the volt¬ 
meters across the d.c. and a c. sides respectively for each of some 
ten values of exciting current obtained from some outside d.c. 
supply, and differing by about equal amounts from 0 to say 25% 
above normal excitation and taking a similar descending set of 
readings. 

(2) By connecting up exactly as in Fig. 168 and driving the 
rotary as a motor from its d.c. side. 

With the field current {a) adjusted to say 20 % above its 
normal value (if possible) start the rotary up in the usual way 
to maximum speed obtainable with this excitation, and li cut out. 
Note the readings of speed (to insure constancy throughout) and 
both a.c. and d.c. voltages for this maximum excitation, and for 
each of about ten smaller values obtained by increasing r and 
differing by about equal amounts down to the minimum prac¬ 
ticable, the speed being kept at the same constant value by 
increasing and tabulate as on page 436. Plot the magnetization 
curve, having exciting currents as abscissae, and the a.c. and d.c. 
voltages as ordinates respectively. 

Deduce a third curve by joining the points obtained on 
deducting the armature drop (= current X its resistance) from 
each of the d.c. voltage ordinates. 

Plot also a curve having a.c. volts as ordinates with d.c. 
volts as abscissae and deduce the voltage ratio of conversion. 
Compare this ratio with the theoretical value and explain the 
reason for any difference. 


ELEGTETGAL ENGINEERING TESTING 


439 


(148) Effect of Variation of (i) Excitation 
( 2 ) Speed on the Voltage Ratio of a 
Rotary Converter (Run from the Direct 
Current Side.) 

Observations. (1) With exactly the same connections as in 
Fig. 168, and with S 2 , open, note the readings of all the 
instruments for constant speed throughout, for about eight 
different values of exciting current (a) differing by about equal 
amounts between the minimum and maximum values possible. 

(2) Repeat (1) with closed, and at l J and full 

load respectively, the load being kept constant by varying 

7 ? 2 ’ ^ 3 - 

(3) Repeat 1 and 2 for a similar variation of speed at 
constant normal excitation and tabulate as on p. 436. 

Plot curves for Tests 1 and 2 at each load having excitino- 
currents (a) as abscissae, and (1) a.c. volts, (2) d.c. amps, (3) 
voltage ratio of conversion, as ordinates in each case, and for 
tests (3) curves having speeds as abscissae with voltage ratio • 
a.c. volts and d.c. amps, as ordinates. 

Inferences. —State concisely all that can be deduced from the 
results of your investigations in the present test. 

(149) Efficiency and Characteristics of Alter¬ 
nating-Continuous Current Rotary Con¬ 
verters. (Run from the Alternating 
Current Side.) 

Introduction. —The investigations to be made in the present 
case are all the more important and instructive because the usual 
application of this kind of rotary, commercially, is to convert a.c. 
to d.c., the machine being supplied with a.c. and running as a 
synchronous a.c. motor. The speed at which it runs will there¬ 
fore solely depend on the number of poles in the field, and on 
the periodicity of the a.c. supply, and if this latter is constant the 
speed will be also, irrespective of the load developed at the d.c. 
side, or of the excitation. As the load increases, for constant 
power factor, a.c. voltage, and frequency, “armature reaction” 
causes an increasing drop of d.c. voltage, and, further, a decrease 
of excitation is necessary to maintain constant power factor, but 


440 


ELECTRICAL ENGINEERING TESTING 


an increased variation of d.c. voltage results. On the other 
hand, a constant excitation increases the intake current, but 
decreases the variation of d.c. voltage. By varying the excitation 
to maintain constant d.c. voltage, this latter, and also the 
efficiency, is increased. In order to minimize the variation of 
d.c. voltage, and maintain a constant voltage ratio as the load 
changes, rotaries are usually compound wound, the series coils, 
just as in the case of an ordinary compound dynamo, producing 
an increase of excitation proportional to the load, and simul¬ 
taneously, the necessary change of power factor. A rotary can be 
over-compounded so as to give an increasing d.c. voltage with load 
to make up for loss of voltage in transmission, in which case unit 
power factor is obtained by field regulation at some fraction of 
full load, thereby giving leading currents at full load. On a light 
constant load, a given variation in the excitation causes a much 
greater change in the current intake and power factor than it 
would do on a heavy constant load. Since the lost armature 
volts in a rotary run from its a.c. side = (current x armature 
impedance), while when run from its d.c. side this quantity 
= current X armature resistance, and also owing to the power 
factor not being unity; to the wave-forms of a.c. supply, and of 
the rotary (run from the d.c. side) being different; and to armature 
reaction, the voltage ratio of conversion will be different when 
run from the d.c. and a.c. sides. Further, when the power factor 
of the circuit is low and the current comparatively large and 
lagging, the supply and lost armature volts will be more nearly 
opposite in phase, and hence a larger armature drop results 
at the small excitations. A rotary has unit power factor at a 
particular excitation, also too low an excitation causes the current 
to lag, while too high an excitation causes it to lead as shown by 
the V curves between excitation and power factor obtained in 
the above investigation. Constant d.c. voltage at all loads can 
be maintained by adjusting the excitation to give unit power 
factor at full load. 

Apparatus.—Precisely that prescribed for Test 146, except 
that the a.c. supply is substituted for, and takes the place of, 
Aj, Ag, Ag, and that some form of synchronizer is needed. If the 
phases are equally balanced, or if all the instruments on the a.c. 
side are unavailable, then any two ammeters such as and 
and any two voltmeters such as Fj, Fg may be omitted. Also one 


ELECTRICAL ENGINEERING TESTING 


441 


of the two wattmeters shown might be omitted, means being pro¬ 
vided by a two-way key for connecting one end of the fine wire coil 
of the wattmeter used to the remaining two supply mains in 
quick succession. It will be noticed that a three-phase rotary is 
assumed for the test, but the same considerations and investigations 
would apply to single- and two-phase rotaries. 

Connections. —To be as shown in Fig. 168, unless modified by 
a reduction in the number of a.c. instruments as mentioned above. 
The simplest and most convenient form of synchronizer to 
employ consists of two ordinary glow lamps supported in two 
ordinary bayonet holders connected in series and carried on a 
base board with two terminals. The sum of the voltages marked 
on the lamp bulbs miost not he less than the sum of the supply and 
converter a.c. voltages. The two terminals of this “ lamp syn¬ 
chronizer ” may be connected so as to short circuit say a piece 
of thin wire short circuiting, say, 

The Process of Synchronizing the rotary with the a.c. supply is 
usually most conveniently accomplished as follows— 

(a) Adjust all instruments which need it, and with /S'g 

open, start the rotary up as a d.c. motor from a d.c. supply E, by 
closing S, and operating the starter (not shown), with which the 
machine is provided, in the usual way. 

N.B. —If there is no starter, the adjustable load resistance R 
may be used for starting up. 

(h) Adjust R so as to obtain the same voltage on as that of 
the main a.c. supply, and then adjust r to give such a speed that 
the lamps go out. The a.c. supply and a.c. voltage of the rotary 
are now equal to, and opposing^ one another, and of the same 
frequency, and hence in synchronism. The switch must 

now be closed, and S at once opened, when the rotary will con¬ 
tinue to run, now as a self-exciting three-phase synchronous a.c. 
motor. 

It may be mentioned that the a.c. voltages of the main supply 
and of the rotary are in assisting series when the lamps show 
steady luminosity, while between this and the “quite out” con¬ 
dition they pulsate in brightness due to the current pulses of the 
two E.M.F.s trying to catch up to one another. 

Of course, a separate motor, or driving source, if available, 
might be used to ruu the pqtary up to synchronous speed instead 


442 


ELEGTRTGAL ENGINEERING TESTING 


of the d.c. supply above named. If the d.c. supply used for 
starting-up purposes is unsuitable for giving the necessary a.c. 
voltage on the rotary at the required speed, the rotary may be 
run up to a much higher speed than that of synchronism, S then 
being opened, and afterwards S^S 2 S^ closed, at the moment when 
the speed falls to such a value that the lamps go out. 

Note. —If an outside d.c. supply is used in synchronizing, then 
directly the rotary is running synchronously on the a.c. supply 
and S is opened, at once disconnect the mains E from the auxiliary 
d.c. supjily, and connect them together to avoid the possibility 
of a future mishap by forgetting to do this at the time. Several 
investigations on the operation of the converter under different 
conditions can now be undertaken. 


Effect of variation of Direct Current Load 
on the operation of the Converter at 
Constant Direct Current Voltage and 
Excitation, and Alternate Current 
Frequency. 

Observations. — (1) Adjust the exciting current {a) by means 
of if) until the intake a.c is a minimum, then with S still open, 
note the readings of all the instruments and the speeds of the 
rotary and generator (giving the main a.c. supply) respectively for 
normal frequency. 

(2) Close S and note the readings of all the instruments and 
the speed, for about eight different d.c. loads rising by about 
equal amounts between 0 and full load, by varying R; keeping 
the current (a), the supply frequency, and the d.c. voltage (F) 
(by varying the excitation of the main generator), constant 
throughout. 

(3) Eepeat (1 and 2) above for a lower and also for a higher 
constant excitation than that previously found in Test 1 above, 
and tabulate your results as shown in the table. 

(4) Plot curves between d.c. output in amps. A as abscissae, 
and (a) efficiency (b) power factor, (c) mean intake a.c. 
amperes, (d) mean a.c. volts, as ordinates in each case. 


ELECTRICAL ENGINEERING TESTING 


443 


(150) Effect of variation of Direct Current 
Load on the operation of the Converter 
at Constant Alternate Current Voltage 
and Frequency, and Direct Current 
Excitation. 

Observations. —(1) With constant normal a.c. voltage and 
frequency maintained throughout, repeat (1—4) Test 149 above, 
plotting for (4d) above the d.c. volts instead of mean a.c. volts 
as ordinates. The curve between V and A is called the d.c. 
characteristic of the rotary. 

(151) Effect of variation of Excitation on the 
operation of a Converter at Constant 
Alternate Current Voltage and Frequency, 
and Direct Current Load (‘‘V” curves). 

Observations. —(1) With S open, and the converter running at 
constant normal a.c. voltage and frequency, note the readings 
of all the instruments, and the speeds of the main generator and 
rotary for about eight different exciting currents (a) between the 
lowest and highest permissible by altering (r). 

(2) Close and repeat (1) for constant loads of about J, ^ 
I and full d.c. load and tabulate as indicated. 

(3) Plot curves between d.c. amperes of excitation as abscissje 
and (a) power factor, (6) intake alternating current, (c) voltage 
ratio of conversion, (d) intake a.c. Watts. 


(152) Variation of Excitation to Maintain 
Maximum Power Factor for Varying 
Direct Current Load at Constant Alter¬ 
nate Current Voltage and Frequency. 

Observations,—(1) With S open, and the rotary running at 
constant normal a.c. voltage and frequency, adjust the excitation 
(a) so as to obtain minimum intake current, and note the readings 


444 


ELEGTRIGAL ENGINEERING TESTING 


of all the instruments and the speeds of main generator and 
lotary. 

(2) Close S and note the value of the exciting current (a) 
necessary to give minimum intake current at about 8 different 
d.c. loads between 0 and the maximum (by altering R), the a.c. 
voltage and frequency being the same at each load. Note all the 
other instrumental readings and tabulate as indicated. 

(3) Plot curves between d.c. load in watts as abscissae and (a) 
d.c. volts, (6) exciting current, (c) a.c. amperes, (cZ) power factor, 
(e) voltage ratio of conversion, (/) efficiency. 

Name . . . Date . . . 

Ili)tary; No. . . . Type . . . Maker . . 

d.c. Side: Volts = . . . Amps. = . . . Speed = . . . 

a.c. Side: Volts = . . . Amps. = . . . Wattmeter Constants = . . . ; A ’'2 = • • . 

Resistances Armature rn = . . . Shunt Vsh = . . . Series Vse = . . . 


Speed of 

Main Alternating Current Supply. 

Gener¬ 

ator. 

Rotary. 

Fre¬ 

quency. 

Currents. 

Voltages. 

Wattmeters. 

Al. 

An. 

^3- 

Vi. 

V 2 . 

n- 

IFi. 

W2. 














Inferences. —Very carefully consider and state all the infer¬ 
ences which can be deduced from the results of your investigations. 

(153) Efficiency and Output of a ‘‘Booster'' 
or of a “ Motor Generator Set." 

Introduction. —It frequently happens in practice that either 
electrical power in one form at a certain pressure is required in 
the same form but at a different pressure, or that electrical power 
of one nature is required in quite a different nature at the same 
pressure or otherwise. The electrical appliance by means of 
which such transformations can be effected is variously termed g, 





























































ELECTRICAL ENGINEERING TESTING 


446 


Motor Generator,” “Booster,” “Rotatory Converter,” “Con¬ 
tinuous Current Transformer,” etc. 

In all these appliances the desired effect is produced by 
machinery in motion, and only so long as it is in motion. The 
motor-driven Booster at the present day is essentially a device 
for transforming direct-current energy from one pressure to 
another. Speaking in general terms a Booster is a machine for 
adding a small percentage of E.M.F. to a large generator and is 
much used* in storage battery systems. The Motor Generator and 
Converter very frequently constitute a device for transforming 
electrical energy in the form of direct currents into that of the 



Fig. 169. 

form of single and multiphase alternating currents or vice versd. 
The motor generator frequently takes the form of two separate 
machines, on the same bed plate, with their shafts in alignment 
and coupled mechanically. One machine M constitutes an electro- 
• motor, fed from an external source of electric supply, the other 
is a generator D which is driven direct by the motor and develops 
electrical power. 

As machines of this nature are used to some considerable extent 
as “regulators ” to “feeder mains,” and also with some slight 
constructional difference as “ equalizers ” on the various systems 
of parallel distribution, the determination of their efficiency and 
output becomes of considerable importance. 

Apparatus.—Motor generator MD ; source of electrical energy 
E ; ammeters -4 j Ag and a ; voltmeters Fj and Fg i rheostats R^ 
(p. 606) and r (p. 599); switches and S 2 ; tachometer. 



















































446 ELECTRICAL ENGINEERING TESTING 

Note.—Prior to starting, all lubricators must be seen to feed 
properly. 

Observations.— (1) Connect up as in Fig. 169, and adjust the 
pointers of all the instruments to zero. Increase 7?2 to their 
maximum and r to a minimum. See that S 2 are open and both 
sets of brushes down. 

(2) Close jS^ and adjust so that the normal speed N for the 
particular “ set ” tested is obtained. Note simultaneously the 
readings of a, Fj, Fg and N. 

(3) Close ^2 and adjust R 1 R 2 that A 2 reads about full 
load current in amperes, N being the same as before. Again 
read all the instruments. 

N.B.—It may be found necessary to vary the excitation of M 
by means of the resistance r in order to keep the speed constant 
throughout any one set of readings. 

(4) Repeat 3 for about ten different load currents A 2 up to the 
maximum, rising by about equal increments at a time. 

(5) Repeat 3 and 4 for speeds 20% above and 50% below normal, 

and tabulate your results as follows— 

Name . . . Date . . . 


Maker of Motor Generator . . . Resistance of Shunt Coil of M (n) = . . . ohms. 

Number of „ ... Type . . . Normal Output = . .. Watts, at... revs, per min. 


'V 

a> 

0 

m 

Shunt Current 
a. 

W M . 

Current 

Ai Amps. 

0 

to 

a 

l-H 

O’ 

> 

r-l 

CQ 

^ ai irH 

• 

Current 

A 2 Amps. 

0 

to 

cd 

0 

> 

JO CO 

P 4 ^ ^ 

N 

!^‘5~ 

Efficiency of 

M otor-generator 

2 = X 100 %. 













(6) Plot an effici^cy curve for each speed having IFg 
abscissae and ^ as ordinates. Also curves having IFj as 
ordinates. 


Inferences.—State clearly all the inferences which you can 
draw from the results of your experiment. Could the combined 
efficiency be increased by any structural alterations in the “ set ” 
tested 1 

(154) Determination of the Periodic E.M.F. 
and Current Curves of an Alternator. 

Introduction.—It may sometimes be desirable to determine 
the periodic curve or wave of E.M.F. and current in an alter- 
























ELECTRICAL ENGINEERING TESTING 


447 


nating current circuit, for the shape of such curves has an 
important influence on the losses occurring in the iron cores of 
any appliances in the circuit. In fact, the more peaked the 
E.M.F. curve, or the more nearly it approximates to a sine or 
even a triangular curve, the less will be the losses occurring in 
such appliances and the greater will be what is called the ^^form 
factor’' 

Two cases may arise in which it is desired to obtain the 
periodic curves, namely (1) when the alternator supplying the 
circuit is at a long distance away, and consequently inaccessible 
in a sense, (2) when the test can be applied close to the alternator, 
if necessary. In either case some convenient form of rotating 
contact maker must be used to close the circuit of a suitable 
measuring arrangement for an instant once every revolution, at 
any definite point in the period of alternation, corresponding to 
the position of the brush or contact arm. Hence by moving this 
contact arm into various angular positions, the periodic curve of 
instantaneous values of varying E.M.F. and current at different 
instants can be obtained throughout the whole period or wave. 
Such a contact maker is illustrated and described in the 
Appendix, p. 619, and in case 1 above it is fitted to and driven 
by a synchronous alternating current electromotor run off the 
supply of which the E.M.F. or current curve is desired. Such 
a motor always runs at a speed bearing a definite and fixed ratio 
to the periodicity of the supply current. In case 2, the one we 
shall here consider, the contact maker is fixed to, and is driven 
by, the rotating portion of the alternator itself. 

Knowing then the periodic curves of E.M.F. and current in a 
circuit together with their phase difference^ at once seen from the 
relative positions of the two curves, the true instantaneous power 
developed in that alternating current circuit can at once be 
deduced. 

The following method of obtaining such curves consists in 
charging a condenser to a certain E.M.F. by periodically 
connecting it, by means of the contact breaker, to the alter¬ 
nating circuit to be tested, and measuring this E.M.F. by an 
electrostatic voltmeter. This will therefore be the instantaneous 
value of the potential at the point of the period of alternation 
corresponding to the position of contact of the brush on the 
contact maker. 


448 


ELEGTEIGAL ENGINEERING TESTING 


The function of the condenser is to maintain the instantaneous 
voltage at a uniform value and so insure a steady reading on the 
voltmeter, notwithstanding the leakage usually occurring from 
it, in the interval between successive contacts. 

If a Kelvin multicellular voltmeter (preferably dead beat type) 
is used for the test it will be necessary to obtain a false zero of 
about 30 on its scale, as the one volt graduations only commence 
from this point upwards. For this purpose an auxiliary voltage 
of about 30, which can be supplied by a battery of about 15 small 
secondary cells, may be used. As the circuit is closed by the contact 
maker for a small fraction of a second only, the condenser should 
have a small capacity, otherwise it may not be fully charged. Even 
though V and C are well insulated a certain amount of leakage 
may occur, depending on the rate of contact— i. e. on the speed of 
D. A known steady P.D. should therefore be applied in place of 
the alternator P.D., and the speed adjusted to the value above, 
then if there is leakage V will read differently when the contact 
is stationary and when periodic; the ratio of these two x Fj gives 
the instantaneous P.D. In the determination of the current 
curve a low non-inductive resistance should be used, so that its 
introduction may not affect the existing conditions of the circuit 
to any appreciable extent. The P.D.s in this case will be small, 
and can be measured by the ordinary “ Null deflection,” or 
balance method on a potentiometer. The amperes per scale 
division can be found by passing a known steady direct 

current through (r) 
from a supply in place 
of E, the rate of con¬ 
tact being as above; 
then known current 
~ scale reading = am¬ 
peres per div. 

Apparatus. —Alter¬ 
nator E with its excit¬ 
ing circuit; contact 
maker AT(p. 619); well- 
insulated electrostatic 
voltmeter V (p. 563), 
and also i to J m.f.cl. 
condenser (7; well-insulated battery B; A.C. voltmeter A.C. 



Fig. 170. 




















ELECTRICAL ENGINEERING TESTING 


449 


ammeter A ; switches S and >5^^; load resistance R, in which the 
current wave is to be found ; non-inductive resistance r, of such a 
size as will carry the current without sensible heating; sensitive 
H.R. galvanometer (p. 569); potentiometer PQ (Fig. 171), with its 
slider key ; E.M.F .—E (2 volts), and reversing key (p. 585). 

Note.—All lubricating arrangements must be seen to feed 
properly when the machines are started. 

E.M.F. Curve.— Experiments — 

(1) Connect up as in Fig. 170. Set JT to 0 on its scale and 
measure the auxiliary voltage {v) on V by short-circuiting C by a 
wire and afterward removing this. 

(2) Start D and adjust its speed to the normal value^say, also 
its excitation so as to give about normal voltage on the volt¬ 
meter Fj. 

(3) With the speed and terminal P.D. constant, note the steady 
reading on F, which therefore is a measure of the instantaneous 
E.M.F. of D for this position of X. 

.(4) Repeat 3 every 20° from 0 throughout the period of 
alternation (360°) by moving A, noting the points when is 
used to reverse the E.M.F. of B. 

(5) Repeat 2-4 when D is giving a convenient constant current 
through a suitable inductive circuit R, 

Current Curve. — Experiments —• 

(1) Connect up as iu 
Fig. 171. Set X to 0 and 
repeat 2 above. 

(2) With R at its 
maximum, close S and 
adjust the current to 
the same value as men¬ 
tioned in 5 above, by 
means of the exciting: 
current. 

(3) With the speed 
normal and terminal 
P.D. constant, obtain 
balance with so that, 
on pressing it, no deflec¬ 
tion occurs on G. Note 

G G 



the scale reading (d) of Aj, which 


























450 


ELECTBICAL ENGINEERING TESTING 


therefore is a measure of the instantaneous P.D. at the resist' 
ance r. 

(4) Repeat 4 above, and tabulate your results as follows— 

Name . . . Date . . . 

Speed of Z> = . . . Revs, per min. Frequency = . . . per sec. 

Capacity of (7= . . . m.f.d.s. Aiix. E.M.F. v = . . . volts. 

Non-ind. Resistance r = . . . oluiis. 

Reading of voltmeler Fi = . . volts Volts per div. of P.Q. (a) = . . 

,, am meter .<4 =. . . am] s. Amps. ,, ,, (/3) = . . . 


Reading of 
X = Phase 
Angle 

e\ 

Total 

E.M.F. 

V. 

Nett Inst. 

E.M.F. 

Vj = V—v. 

Reading 
of P.Q. 
id). 

Actual Inst. 
E.M.F. atr. 

(a d). 

Actual Inst. 

Current 
through r. 

1 o-d 

Aj — — =pd. 

Angle of 
Phase 
between 

Fj and Aj. 









Note. —2 V must either be added or subtracted, on reversing the 
battery switch, according to whether (v) was previously in helping 
or opposing series. Half the difference between the readings = v. 

It will be obvious that the instantaneous terminal voltage of 
D can be obtained in addition to the current, if in Fig. 171 E 
has a value at least = the maximum value of that voltage; PQ has 
a high resistance, and a two-way key be used in place of the 
permanent connecting wire between C and r, so that it will 
connect G successively to (r) as shown, and to the junction of 
R and S, which latter gives the terminal instantaneous voltage. 

(5) Plot the E.M.F. and current curves on the same curve 
sheet, having 0° as abscissae in each case and and A-^ 
as ordinates respectively, and calculate out from them the 
\/mean square of the instantaneous values of V and A. 

In determining the periodic curves of E.M.F. in a high tension 
circuit a potentiometer arrangement should be used; the wires 
going to P (Fig. 170) being connected to the ends of a suitable 
known fraction of a known high resistance placed across the 
mains, and which can carry an appreciable current, say ^ to | an 
ampere, thus illuminating any error arising from the capacity 
current of the voltmeter. 

(155) Determination of the Periodic E.M.F. 
and Current Curves of an Alternating 
Current Circuit. (Ballistically.) 

Introduction. —Should an electrostatic voltmeter not be avail¬ 
able as in the preceding method, a reflecting ballistic galvanometer 
















45J 


ELECTRICAL ENGINEERING TESTING 

G may b© used, and it sliould preferably be of the moving coil 
D Arsonval type, so as not to be affected by the stray magnetic 
fields invariably met with in a dynamo-room. In all other 
respects the present method is precisely similar to the last. is 
a reversing key (Fig. 255) for obtaining deflections each side of 
zero. AVhen the two-way key K (Fig. 25G) is put to stud 1 the 
condenser 0 is charged to an E.M.F. corresponding to that for the 
position of the contact arm X in the period of alternation. On 
putting X to 2, C is discharged through G, and since the resulting 
throw is CO the quantity flowing out of C, wliich in turn is 
QC 6’ X F, where C = capacity of the 
condenser and V the E.M.F. to which 
it is charged, we see that the resulting 
momentary throw on G jc: F, since 
the capacity is a constant. 

This should be larger the smaller 
the charging E.M.F. to be measured 
The current curve is obtained in a 
precisely similar manner to that indi¬ 
cated in Fig. 171. 

(156) Delineation of Wave-Forms by means 
of the Duddell Oscillograph. 

Introduction. —The shape of wave-forms in general, but 
perhaps more especially those of current and pressure in alter¬ 
nating current circuits, are of the utmost importance to electrical 
engineers. For instance the efliciency of transformers and a.c. 
motors, and even the working of the latter, is in some cases 
seriously affected by the wave-form of the supply. On the other 
hand, the optical efficiency of the a.c. arc has been found to be 
44% higher with a flat-topped E.M.F. curve than with a peaked 
curve, while transformers work most efficiently on peaked curves. 
Again the wave-form reveals the presenceror otherwise of higher 
harmonics due to accident d though often avoidable resonance 
effects, so dangerous in causing the breakdown of the insulatioa 
of high-tension electric cables. 

The oscillograph itself consists of a highly specialized reflecting 
d’Arsonval galvanometer having an extremely small periodic 





















452 


ELECTRICAL ENGINEERING TESTING 


time, when undamped, of from to ^ second, 

depending on the type of instrument. 

For all ordinary frequencies the oscillograph is perfectly dead¬ 
beat, absolutely free from hysteretic errors, and has practically 
no self-induction or capacity. 

It is therefore an accurate instantaneous ammeter or volt¬ 
meter capable of giving a deflection which is at any moment 
accurately proportional to the instantaneous value of the variable 
even with frequencies of 300 or more periods per second on 
any wave-form whether periodic irregular, or non-periodic and 
whether continuous or alternating. 

Thus in addition to recording E.M.F. and current waves, such 
an instrument will indicate the charge and discharge curves of 
condensers, the changes of P.D. and current on breaking an 
inductive circuit, the P.D. and current changes in the armature 
coils of a dynamo or motor, or in the primary of an induction 
coil or even the very rapid changes when the d.c. arc hisses. 
Doubtless it will be employed for a vast number of other 
determinations as time and necessity arise, but the preceding 
merely serve to indicate some of the uses to which this highly 
important instrument has already been put. 


Construction of the Duddell Oscillographs. 

The apparatus consists of the galvanometer, combined either 
with a rotating or vibrating mirror, moving photographic film, 
or falling photographic plate. 

Fig. 173 is a diagrammatic view of the galvanometer part of 
the instrument showing the principle on which it works. In 
the narrow gap between the poles iV, aS^ of a powerful magnet 
are stretched two parallel conductors s, s formed by bending 
a strip of phosphor-bronze back on itself over the pulley P 
which is attached to a light spring balance. At the bottom 
ends the strips are clamped on a block, K, while at the top they 
are held in position by the bridge piece L. By altering the 
tension on the spring stretching the phosphor-bronze loop, the 
periodicity of the instrument can be varied at will. Each strip 
or leg of the loop passes through a separate gap (not shown) in 
the magnetic circuit. The clearance between the sides of the 



ELECTRICAL ENGINEERING TESTING 


453 


gaps and the moving strip is but 0’38 mm., and these gaps are 
filled with a viscous oil, over which is placed a small lens, which 
is held in position entirely by the surface tension of the oil, and 
serves in its turn to keep the oil in place. The object of the 
oil is to damp the movements of the strips. A small mirror 



Fig. 173.—Essentials of the Vibrating System. 


marked M is attached to the loop, as shown. The effect of 
passing a current through one of these loops is to cause one leg 
of it to advance whilst the other recedes, and the mirror is thus 
turned about a vertical axis. In the high frequency instrument 
the natural period of vibration of the loop is ioooo ^^ ^ second, 
and the clearances being, as stated, extremely small, the damping 
effect of the oil is so great, that the instrument can be relied 
























454 


ELECTRICAL ENGINEERING TESTING 


upon to give accurate results even when the periodicity of the 
current to be tested is over 300 periods per second. Small fuses 
below the loops protect these from injury in case of accidental 
excessive cuirent. The fuses consist of very fine wires enclosed 
in glass tubes, which are held in position by spring clamps. 

The beam of light reflected from the mirror M is received on a 
screen or photographic plate, the instantaneous value of the current 
being proportional to the linear displacement of the spot of light 
so formed. With alternating currents the spot of light oscillates 
to and fro as the current varies and would thus trace a straight 
line. Hence to obtain an image of the wave-form, it is necessary 
to traverse the photographic plate or film in a direction at right 
angles to the direction of movement of the spot of light. A 
second mirror can be interposed in the path of the beam of light, 
and this mirror caused to vibrate or rotate so as to impart to the 
beam of light a uniform motion proportional to time in a plane 
at right angles to the plane of vibration of the beam due to the 
current. The spot of light will now trace out on a stationary 
screen or plate the time curve of the variation of the P.D. or 
current as the case may be. If the variations are periodic, as in 
alternating currents, then the second mirror can be synchronized 
and the spot of light caused to trace out the wave-form over and 
over again. 

The various methods of examining and recording the wave¬ 
forms will be described later. 

The Oscillograph is provided with an adjustment for slightly 
increasing the periodic time and sensibility. This may be done 
by altering the tension of the strips. This is not advisable, 
however, as it is liable to spoil the definition of the spots. 

Four standard types of Duddell Oscillograph are made and are 
in common use, namely— 

Type I. The Double High Frequency Oscillograph, which is the 
most sensitive type and has a powerful electro magnetic field. The 
magnetizing coils are wound in eight sections, and by suitably 
connecting them the field can be excited direct from any voltage 
between 25 and 100 volts or from 200 volts with an 8-c.p. lamp in 
series. The magnetic circuit being saturated,a change of 4% up or 
down in exciting current only changes the sensibility about 1%. 
To introduce damping oil, the vibrator cover should be removed, 
the lens lifted up, the vibrator being held horizontal and a drop 


ELECTBICAL ENGINEERING TESTING 


455 


or two of the special oil placed on the gaps over the mirrors. 
The lens is then lowered into place, the lower edge first, care 
being taken not to imprison any air bells. The temperature of 
this oil is measured by inserting the bulb of a thermometer at the 
back of the vibrator. 

II. The Single Permanent magnet, and III. the Double Per¬ 
manent magnet. Oscillographs are similar to one another. In 
each the electro magnet of Type I. is replaced by a permanent 
magnet, and the damping oil, which is introduced by means of a 
small cup at the back of the instrument, is adjusted to give 
correct damping at 15° C., and practically correct damping 
betwen 10° 0. and 20° C., so that no thermometer is needed. These 
two types are portable and can be easily insulated for research on 
10,000 volt circuits by placing it on an ebonite table. 

IV. The Double Projection Oscillograph is somewhat similar 
to Type I. and is most suitable for teaching and lecture work. 
Wave-forms having a total amplitude of 1 metre can be thrown 
on to a screen. 

The above types are single or double according as they possess 
one or two strips respectively. The double or two-strip pattern is 
practically two single instruments built compactly in one magnetic 
field and capable of recording simultaneously any two distinct 
wave-foims. A fixed mirror is fitted in addition to give the 
datum or zero line. Types II. and III. can be arranged in 
portable form. 


Three Methods are generally employed for 
Observing and Recording the Move¬ 
ments of the Spots. 

1. Visual observation. A rotating mirror is placed with its 
axis horizontal in such a position that the reflection in it of the 
moving spot on the screen can be examined; when, owing to 
persistence of vision, the moving spot will appear drawn out 
into a bright time curve of the variations which it is required to 
observe, and this curve can be sketched if required for future 
reference. 

2. Recording by photography. A photographic plate or film 


456 


ELEGTBIGAL ENGINEERING TESTING 


is caused to move rapidly at right angles to the plane of vibra¬ 
tion of the beam of light, so that the moving spot traces out on 
it the required variations. 

The photographic method is very expeditious, and gives permanent 
records which are free from all errors of a personal nature ; it is 
the only satisfactory method of recording irregular non-periodic 
variations of P.D. and current. 

3. Tracing. In this method, which is only applicable to 
periodic variations, the beam of light from the Oscillograph is 
reflected by an additional mirror with its axis horizontal before 



Fig. 174.—Tj’pe I. Double Oscillograph with Synchronoa.s Motor and 

Tracing Desk. 


reaching the screen, and this latter mirror is caused to move 
uniformly and synchronously with the period of the variations 
to be recorded. This combination of the two motions at right 
angles, the one proportional to the instantaneous value of the 
current through the Oscillograph, and the other to the time, 
causes the spot to travel continuously along the time curve of 
variation of the current, which curve, if the frequency is suffici¬ 
ently high, will appear as a stationary bright line of light. This 
curve may be recorded by tracing or photography. In the Pro¬ 
jection Oscillograph, this stationary line of light can be thrown 
either on the screen or on the tracino: desk. 

The arrangement employed for tracing a.c. wave-forms which 
remain fairly constant in shape and frequency and thus obvi¬ 
ating the necessity for using photography is shown in Fig. 174. In 


























































































































ELECTRICAL ENGINEERING TESTING 


457 



it the light from the Oscillograph mirrors is reflected vertically 
by a small mirror which is made to vibrate synchronously by 
means of a specially designed alternate current motor. The light 
is thrown on to a curved screen, on which tracing paper is held 
by means of a clip and on which the wave-forms appear as 


* Fig. 175.—Type III. Double Permanent Maguet Oscillograph. 

stationary curves of light, and may be traced by hand. The 
small mirror is vibrated by means of a cam attached to the 
motor shaft. The cam is so arranged that the mirror moves 
uniformly for about 1J complete periods during which the wave¬ 
form is observed ; it then returns rapidly to its starting point 
during the remaining J period. During the half period of return 
motion, the light is cut off from the Oscillograph by means of a 
sector fixed to the motor. 




































458 


ELEOTBWAL ENGINEERING TESTING 


The Oscillograph shown in Fig. 175 can of course be substituted 
for that shown in Fig. 174, the tangent screw heads s, s, Fig. 175, 
being for the purpose of bringing the spots of the Oscillograph 
to zero on the screen. 

The Synchronous Motor. —Seeing that this must run dead 
synchronously with the wave-forms to be recorded, it should be 
supplied from the same circuit. 

I'o start the latest type of motor, connect the three terminals on 
it to the phase-splitting board by means of the three-way flexible 
lead attached to the latter. Care must be taken to connect the 
wires to the terminals correspondingly marked, the remaining two 
terminals on the phase-splitting board being connected to the 
a.c. supply having a P.D. of 100 to 120 volts with any frequency 
between 30 and 100 per sec. 

See that the armature of the motor is quite free by turning 
the milled head, and that the bearings are well oiled, then after 
pushing the movable core of the choking coil in as far as it will 
go, close the switch. Now give the armature a start by sharply 
twisting the milled head on its spindle at the vibrating mirror 
end in the clockwise direction, when it should continue to run 
and increase in speed up to synchronism. 

If the motor does not attain synchronism (indicated by the 
“ beats ” in the sound emitted), draw out the core of the choker 
little by little until a constant rhythmic hum is given out. 

The position of the core for synchronism depends on the 
wave-form and frequency, being further out the higher the latter. 
If the core is too far in, the motor will not attain synchronism, 
and if too far out the motor will take too much current and get 
hot. 

It might be necessary to remove the load from the motor at 
starting by pressing back the mirror so as to lift the “follower” 
of the cam. 

The wave-forms will have more than one complete period, and 
will move to and fro on the tracing disc or screen if the motor 
is running below synchronous speed. 






ELECTRICAL ENGINEERING TESTING 


459 


Method of Connection of Oscillographs to 
the Circuits to be Investigated. 

I 

Ihe P.D. required to work the Oscillographs when fuses are 
used in series with the strips is only 1 to 1*5 volts. For current 
curves a non-inductive shunt should be placed in the main 
circuit and connected as shown in Figs. 176 and 177. The low 
resistance R 2 serves to adjust the sensibility to a round number 
of amperes per millimetre. An Ayrton-Mather shunt giving six 
sensibilities for currents from 1*5 to 60 amperes is made for this 
purpose. When the sensibility is adjusted by altering R^ to a 
round number of amperes per millimetre for any one of the 



Fig. 176.—Diagram of Conuection of Oscillograph to Low Tension Circuit. 


sensibilities, all the other positions of shunt are simple multiples 
of the same; thus with Oscillographs Types L, II. and III. the 
sensibilities may be 0*05, 0*1, 0 2, 0*5, 1*0, 2*0 amperes per milli¬ 
metre. Standard Potentiometer shunts constructed for a drop 
of 1 to 1*5 volts can also be used in place of A 3 . 

For P.D. measurements up to 250 volts a non-inductive 
resistance Aj is placed in series with the strips, Fig. 176, which is 
adjusted to give a round number of volts per millimetre deflec¬ 
tion. The switches Aj, Ag and fuses y’ 1,/2 should be arranged as 





















460 


ELECTRICAL ENGINEERING TESTING 


shown in the diagram so that no P.D. can exist between the P.D. 
and current strips due to their action. 

For P.D.s up to 15,000 volts the arrangement shown in Fig. 
177 is much safer, consists of several specially wound 10,000 
ohm resistance frames joined in series and giving about 7 to 10 
ohms for each volt, so that on a 10,000 volt circuit R^ would be 
70,000 to 100,000 ohms; is a 100 or 200 ohm coil forming with 
a potential divider ; is a resistance to adjust the sensibility 





I 


b 


Fig. 177.—Diagram of Connection of Oscillograph to High Tension Circuit. 


to a round number of volts per millimetre deflection. The same 
resistance box as is used for 7?^, in Fig. 176, is suitable. When 
P.D. curves only are being recorded then should be connected 
at that point in E^ which has the least potential above earth. 
All the resistances E^, J? 2 > -^4 ^5 ’^aust be insulated so as 

to stand four or five times the working voltage to earth. Unless 
the side of the circuit containing E^ is permanently connected to 
earth it is better not to use any switches or fuses in the Oscillograph 
circuit as shown, and permanent magnet Oscillographs Types Nos. 
II. or III. should he used. It is also advisable to connect one 
terminal of the strips to the frame so as to screen it from 
electrostatic effects. On high voltage circuits the synchronous 
motor if used is best supplied through a suitable transformer. 

























ELECTRICAL ENGINEERING TESTING 


461 


When using the double Oscillographs the two pairs of strips should 
he so connected to the circuit that it is impossible under any circum¬ 
stances that a higher potential difference than 50 volts should exist 
between one pair of strips and the other^ or between either pair and 
the frame. 

The Oscillographs should be calibrated with continuous 
currents and the resistances R-^y R^ adjusted so that orre mm. 
deflection corresponds to a convenient number of amperes or 
volts, as the case may be. 

The following table gives some useful approximate data for 
the various forms of Duddell Oscillographs. 

Table VII. 




Single 

Double 



Double High 

Peimanent 

Permanent 

Double 


Frequency 

Magnet and 

Magnet and 

Projection 


Oscillograph. 

Portable 

Portable 

Oscillograph. 


Oscillograph. 

Oscillograph. 


Resistance of Field 





Coils in series at 

80" C. . . . 

360 ohms 



650 to 700 ohms 

Normal Exciting 





Current with the 
Coils in series 

0’28 ampere 



0 '27 ampere 

Working Tempera- 





ture for corre< t 





damping with the 
oil supplied . 

25" C. to 35' C. 

10' C. to 20' C. 

10° C. to 20° C. 

30' C. to 35 C. 

Normal Tension on 




6 lbs. 

strips . 

Periodic Time (un- 

8 ozs. 

1^ ozs. 

2^ ozs. 


damped) with the 
above tension 

gAo to 10X00 

30’oiJ to jo^jo 

lo'cn to sdXjo 

tbW to 20X30 


sec. 

sec. 

sec. 

sec. 

Normal Scale dis- 




300 cms. 

tance . 

50 cius. 

50 cms. 

60 cms. 


(25 cms. when used as Portable 




Oscillograplis) 


Sensibility with the 



. 


above tension, 

normal exciting 
current and scale 
distance with 

damping oil in 
instrument . 

290 mm. per 

320 mm. i^er 

290 mm. per 

50 cms. per 


araiiere 

ampere 

ampere 

ampere 

Normal Working 





Current in strips 
for alternate cur¬ 
rent wave-forms . 

0-05 to 0-10 

0-05 to O'lO 

0-05 to 010 

0*5 ampere 


ampere 

amijere 

ampere 


Resistance of strips 





without fuse and 
connections. 

about 5 ohms 

about 5 ohms 

about 5 ohms 

about 1 ohm 

Do. do. with one 





fuse and connec¬ 
tions . 

about 10 ohms 

no fuses 

no fuses 

about 1"6 ohms 





























462 


ELECTRICAL ENGINEERING TESTING 


(157) Determination of the Efficiency of an 
Electro-Motor-Fan Set. 

Introduction. —The very extensive use to which the electrically 
driven fan is put, coupled with the large number of combinations 
of different types of motors and fans in use at the present day, 
makes it desirable to obtain some measure or gauge of the 
efficiency of such an electric fan as an air circulating device. 

From a commercial point of view, the utility of a fan can best 
be judged by knowing (1) the quantity of air, reckoned say in 
cubic feet, passed by the fan in unit time, say 1 minute ; (2) the 
power required to drive the fan to give this. Also, from a 
mechanical point of view, the speed of the fan under these 
conditions, and, as a matter of scientific interest, the pressure 
of the air thus passed. 

In order to carry out the test, it will be necessary to provide 
the fan with an air conduit for the purpose of restricting the 
current of air to a definite path. This may very simply and 
conveniently consist of a tubular casing of either circular or 
square cross section, open at both ends, and from three to four 
diameters long. The fan must just fit into one end, and any 
crevices due to loose fitting between the sides of the conduit 
and fan must be filled in air-tight, so that the only path for the 
air is through the fan itself. 

The determination of the pressure of the draught through the 
conduit CD, but which, however, is not essential to the test, and 
merely of interest, can be determined by a special form of water 
or spirit gauge G. 

Since the pressure of the draught is very small, an ordinary 
vertical U pressure tube would hardly indicate it, and not 
sufficiently accurately for any practical use. To measure such 
small pressures, the gauge, in the present instance, may con¬ 
veniently take the form of a slanting U glass tube about to 


ELECTRICAL ENGINEERING TESTING 


463 


^ bore, one limb L of which is extended and*bent into the form 
shown at L 0, the end 0 being bent so as to face the incoming 
draught of air from the end C of the conduit. 

If now this U tube contains coloured alcohol, any slight 
pressure at 0 will cause a difference in level of the liquid in the 
two limbs, the vertical distance between the ends of the columns 
being a measure of the pressure at 0.. Thus even though this is 
very small, the end of the liquid column in either limb may have 
moved through a considerable distance, which increases as the 
angle of slope {&) gets smaller. Hence if the tube is provided 
with a scale and calibrated, it can be made to read small fractions 
of an inch pressure of water, etc. 



Fig. 178. 

A little cotton wool placed in each end of the tube helps to 
damp the motions of the liquid column caused by rapid fluctuations 
of air pressure. 

In order to obtain the volumetric discharge of air for any 
particular speed of the motor M, the velocity of the propelled 
air must be obtained by means of a rotatory anemometer placed 
in different positions in the. cross section of the conduit some 
little distance from the end C, the instrument being supported 
at the end of a stout wire or rod so as not to alter the flow 
of air. From the various readings a mean may be obtained for 
the wdiole section. 

Now let V = mean velocity of the air in feet per minute as 
given by the anemometer, and let s = area of cross section of the 
conduit in square feet, then the volume of air discharged per 
unit time, i'.e. number of cubic feet of air per minute = vs; this 
will vary with the speed of the motor. 
























464 


ELECTRICAL ENGINEERING TESTING 


Let tv = weight in lbs. of 1 cubic foot of air at the barometric 
pressure and temperature of the room in which the test is 
made (see table, p. 650), then W = wvs = the weight in lbs. of air 
discharged per minute; but the kinetic energy of a mass [m) lbs, 
moving with a velocity v feet per second = foot-poundals, 

and this is a measure of the work done. 

Hence the work done on the air = g foot lbs. 

i WVS / 1 

/. H.P. developed = 32-2 x 33000"" 2 x 32*2 ^ W/ ^ 33000' 

Apparatus.—Motor if with its attached fan Fto be tested ; suit¬ 
able conduit CD ; anemometer ; air pressure gauge G ; voltmeter 
F; ammeter i.; switch K] rheostat R (p. 606) and source of electri¬ 
cal supply E\ speed indicator for obtaining the speed of the motor. 

Observations. —(1) Connect up as shown in Fig. 178, and 
adjust the instruments to zero if they require it. See that all 
lubricating cups feed slowly and properly. 

(2) With R at its maximum, close A, and adjust the speed 
of the motor if to the lowest convenient amount, then note the 
readings of V, A, speed, anemometer, and gauge G. 

(3) Take the anemometer reading at different positions in the 
cross section of the conduit at this speed of M, and take the mean. 

(4) Repeat 2 and 3 for twelve or fifteen different speeds of the 
motor, rising by about equal increments to the maximum per¬ 
missible, by varying the rheostat R^ noting simultaneously the 
readings of all the instruments at each speed. Tabulate your 
results as follows— 


Name , . Date . . . 

Electro Motor: No.. . . Type . . . Maker . , . Normal: Volts. . . Amps. . . . Speed . . . 

Fan : No. ... ,, . . . ,, ... No. of blade.s . . . 

Sectional Area of Conduit s = ... sq. ft. Weight of 1 cubic foot of air at time of test = ... lbs. 


u 

OP 

. 





Horse Tower 




d 

^ . 







Mean anemorne 
reading {v) 
ft. per min. 


.£ 

0) r 

o a 

a ® 



73 ^ 

tl 

1 


p.S 
m ■e 

p-l p 

c3 

u 

Q 

M Cl. 

c3 

o 

Ui ^ 

o 

Speed of 
revs, pe 

09 

o 

> 

in 

P, 

g 

a> to 

1 « 

Ph 

O 00 

> ^ 
a> 

o 

rH 

X 

(M 

O 

lO 

C - O 

.5 3 2 

5B 11 

(a 









t'- • 


1 






























ELECTRICAL ENGINEERING TESTING 


405 


(5) Plot the following curves on the same curve sheet having 
(a) the speed in revolutions per minute of the motor, (b) H.P. 
absorbed by the motor as abscissae, with the volume of air dis¬ 
charged per minute as ordinates in each case; also between 
(a) as abscissae with (c) the mean velocity of the air draught 
as given by the anemometer, (d) the H.P. absorbed in the 
transference of air as ordinates in each case; lastly, between 
efficiency as ordinates and speed as abscissae. 


(158) Determination of the Commercial Effi¬ 
ciency of a Gas Engine-Dynamo Gener¬ 
ating Set. 

Introduction. —It is most desirable that any generating unit 
such as the above should be tested at various loads or electrical 
outputs in order that the best running conditions may be 
discovered and the performance generally of the unit observed. 

; This is done by “ indicating ” the engine at the various 
electrical load outputs desired, and so finding the relation between 
the total power exerted on the piston of the engine, commonly 
termed its indicated horse-power (I.H.P.), and the corresponding 
power utilized or developed by the generator in the external 

circuit, which may be reckoned in ' electrical horse-power 

(E.H.P.). 

1 In order that the cost of running at any given electrical 
output per hour (say) may be determined, it will be necessary 
to measure the volume of gas used in the engine, which can 
usually be done easily enough, as nearly all gas engines are 
provided with separate gas-meters on the inlet pipe of the engine. 
Should, however, this not be the case, the test must be made 
when all gas-jets are out and the readings on the main meter 
recorded. 

The jacket water, or volume of cooling water passed through 
the water jacket of the cylinder of the engine, should be measured, 
and this can best be done by a water-meter inserted in the inlet 
water-pipe. If such a meter is not available, a fairly large tank 
can be used (the volume of which can be calculated) to supply the 

H H 



406 


ELEGTBIGAL ENGINEERING TESTING 


water jacket, then the time taken to empty the calculated known 
volume of water wdll enable us to get what is required. If the 
temperature of the inlet and outlet water of the jacket is taken, 
the heat removed from the cylinder in thermal units can at once 
be deduced, knowing the volume of water passed in a given time. 

The engine must be provided with a stop-cock in communica¬ 
tion with the interior of the cylinder, and into the outer end of 
which the nipple of the indicator is screwed. This indicator may 
be of the Richards’ type, and, if the gas engine is running at a 
high speed, the indicator should be a high speed one. If the 
diagrams taken by this indicator in such cases are not shortened, 
a stronger drum-spring will be needed to get over the effects of 
inertia in the drum which carries the card. 

If the engine has an ordinary double (Otto) cycle and gets the 
maximum number of explosions possible, which would occur 
when it is running at or near full load, then the speed in revolu¬ 
tions per min. -r 2 will give the number of explosions per min. 
If, however, the gas engine is running on light loads it will 
“ miss ” an explosion frequently, in which case, since the number 
of such per min. is a factor of the I.H.P., they must be counted 
separately, either mentally or automatically by an attachment 
or counter actuated by the inlet gas-valve lever. 

Furthermore, in order to obtain the total cost of electrical energy 
delivered at the switch-board, we must know the amount of oil, 
cotton waste, wages, and interest on depreciation and first cost of 
the plant for the period over which the run extends; these items, 
however, pertain merely to what we may term the economic 
efficiency of the plant. 

The electrical power develoj)ed by the dynamo can be taken up 
either in the apparatus on the circuit to be supplied, or in 
suitably designed water rheostats having ample plate area to 
avoid variations in the output. This may take the form of a 
rectangular water-tight wooden trough, having a fixed zinc, iron, 
or copper plate at one end, connected to one terminal of the 
dynamo, and a similar movable plate, capable of being moved to a 
considerable distance from the fixed plate, thus enabling the 
current output to be varied; this plate is perforce connected to 
the other terminal of the generator. 

The mean effective pressure P in lbs. per sq. in. during the 


ELECTRICAL ENGINEERING TESTING 


467 


explosion, which is required in the calculations, is calculated, 
from the indicator diagrams taken, in the manner described on 
pp. 471 and 530. 

Note. —The effective area of each plate of the water rheostat 
should be something like 4—9 sq. ft. for currents of about 500 
amperes. 

Apparatus. —Gas engine-dynamo set D to be tested, of which 
the gas engine is not shown; indicator (p. 531) and reciprocating 
lever gear for rotating the 
card drum; tachometer; 
voltmeter V ; ammeter A ; 
trough rheostat T for ab¬ 
sorbing the electrical output 
from E. 

, In addition a “ cut-out 
and switch are desirable in 
the main circuit, and a water- 
meter with necessary separate gas-meter for the engine. 

Observations.—(1) Connect up the electrical circuit as in 
Fig. 179, and adjust all instruments to zero. Fill :7^with water in 
which a few handfuls of washing soda have been dissolved and 
set the plates at the extremities of the rheostat. 

I N.B.—The plates may be provided with massive terminals for 
connection to the main leads, otherwise the ends of these latter 
should be spread out, fan-wise, and soldered to the plates. 

■ (2) Measure approximately how much oil is required to Jill all 

lubricators in use, which must be set to feed properly just before 
starting the trial. 

' Insert the most suitable spring in the indicator and a card on 
the drum, then screw the indicator to the cylinder cock and 
connect to the reciprocating gear. 

(3) Start the “set"’' up to its normal speed and take an 
indicator diagram from the engine with E on open circuit. 

(4) At a noted instant simultaneously read all the instruments 
and meters, then quickly switch on and adjust A to full load and 
take an “ engine card ” again. 

I Note. —^At least four observers will be required for the 
trial. 

(5) Simultaneously read all the appliances every twenty minutes 

















468 


ELECTRICAL ENGINEERING TESTING 


throughout the trial, which should last at least three hours, 
taking a “card’' at each. 

(6) Repeat 2—5 for J full load on E if possible, and tabulate 
your results as follows— 


Name. . . 


Date . . . 


Gas Engine; No. . . . Makers . . , Type . . . Normal speed = . . 

Piston: Area (o) = . . . sq. inches. Stroke (i) = . . . feet. 
Dynamo : No. . . . Makers .. . Type .. . Normal: Volts = ... Amps. = . 

, rE.ii.p. 

Indicator: No. . .. Type . .. Scale of Spring used ... Mean j ^ p - 


revs. per min. 

. Speed = ... 

= • • • \ during 
: ... / trial. 


Time 

Output from 
Dynamo. 

Gas Engine. 

Efficiency of “ set ” 

= mean 

I.H.P. 

Gas. 

Water. 

I.H.P. calculated 

from cards . 

33000 

of observations. 

in Hours from 
Start. 

QQ 

• 4 ^ 

0 

> 

Amps. A. 

Ico 

li 

w 

Explosions per 
min. (n). 

Meter reading. 

Total Vol. used 
in trial. 

Cubic feet 
per mean. 

Meter reading. 

Total Vol. used 

in trial. 

Gallons 

per 

mean. 

I.H.P. 

E.H.P. 

I.H.P. 

E.H.P. 


















Inferences. —^W^hat inferences can be deduced from the results 
of the trial 1 

Calculate the total cost per E.H.P. hour delivered at the 
switch-boards, taking the approximate average costs of the various 
factors. 


(159) Determination of the Commercial Effi¬ 
ciency of a Steam Engine - Dynamo 
Generating Set. 

Introduction. —We have already described in detail the usual 
methods of determining the efficiency of both direct and alternat¬ 
ing current generators without reference to any prime mover 
such as a steam, gas, or oil engine. The common practice, how¬ 
ever, at the present day, of employing “ direct coupled sets ” in 
central stations, consisting of the generator placed on, and fixed 
to, the same bed-plate as the engine and coupled direct to it, 
makes the test of the performance of such a combined generating 
set one of extreme importance. 

This practice has resulted from the endeavours of central 











































ELEGTBIOAL ENGINEEBING TESTING 


469 


station engineers to curtail the amount of floor area required for 
a given station and to avoid the loss of power and trouble in¬ 
separable from driving by belts and ropes. In order to determine 
the combined efficiency of a generating unit, whether direct 
coupled or otherwise, we require to measure the useful or nett 
electrical output, which can be done with the aid of an ammeter, 
voltmeter, and one or more suitable rheostats in the manner 
described in the earlier pages of this book. 

In addition we must know the gross or total H.P. exerted by 
the piston of the engine, usually termed the Indicated H.P. 
This can be determined by aid of the engine indicator j for the 
detailed theory of which the reader is referred to special “ works 
dealing with such indicators almost entirely. There are, however, 
some important details connected with the use in general of the 
various forms of these instruments, which may with advantage 
be mentioned here, but otherwise it will be assumed that the 
theory, construction and action of the engine indicator is 
understood. 

Belation between length of indicator diagram and engine speed ,— 
It must be carefully remembered that as the paper drum of the 
indicator is rotated by a reciprocating motion from the piston or 
cross-head of the engine, its inertia at the higher speeds may in¬ 
troduce errors in the diagram; in other words, its motion may 
continue, from this cause, beyond what actually represents the 
true driving motion. To minimize such an error the angular 
motion of the drum must be reduced, and consequently a shorter 
diagram obtained as the speed increases, in order to insure a true 
and properly proportioned diagram. 

In this connection it may be noted that for speeds up to 200 
revs, per min. the driving of the drum should be so arranged that 
its angular motion gives a diagram about long, and this 
should be made to diminish almost inversely proportional to the 
increase of speed. Since in addition increase of speed will require 
a stronger piston spring, the height and length of diagram will 
decrease in about the same proportion for increase of speed, thus 
giving a properly proportioned and accurate diagram. 

Gearing of Engine Indicators. —In order that an accurate 
diagram may be obtained it is all-important that the reducing- 
gear for driving the drum should reduce the piston motion in 


470 


ELEGTRIGAL ENGINEERING TESTING 


exactly the same proportion at any and every part of the stroke. 
For such gears the reader is referred to works dealing with the 
subject of engine indicators. In the use of the indicator great 
care should be taken to keep it free from all dust and well oiled 
with watch oil, as the least friction in the cylinder or multiplying 
levers may cause a distortion of the diagram, the boiler pressure 
and engine speed will determine what strength of spring is to be 
used in the cylinder, and the lighter this is, the higher the 
diagram and the more accurate the measurement, providing 
inertia effects are absent. 

The engine piston must of course never be allowed to close the 
outlet pipe between the engine and indicator cylinders, hence the 
latter should be screwed on to a cock at the end of the engine 

cylinder. For accurate 
work diagrams must be 
taken at both ends of 
the cylinder. Fig. 180 
represents the approxi¬ 
mate shape of a dia¬ 
gram which would be 
taken from an ordin¬ 
al ary non-condensing en¬ 
gine. The indicator is 
0 first made to draw the 
straight line 0'A\ done 
by putting both sides 
of its piston in communication with the atmosphere by opening 
the stop-cock so as to cut off all steam from the cylinder of the 



engine and allow the air to enter underneath the indicator-piston. 
The pressure both sides of this latter will thus be the same and 
equal to that of the atmosphere, consequently if the pencil be 
lightly pressed against the moving card, the horizontal straight 
line 0 A , termed the atmospheric line, will be described. OA is 
the absolute zero line, parallel to O'A', and drawn below it at a 
distance representing, to the scale of the diagram, the atmo¬ 
spheric pressure at the time of the test. 

In Fig. 180 j5 is the point of admission of steam to the engine 
cylinder, D the point at which the slide-valve begins to close, E 
the point at which it is quite closed. From E io F the admitted 









ELECTRICAL ENGINEERING TESTING 


471 


stecam is expanding, and at F the release begins, being completed 
at G. The exhaust valve begins to shut at H and is quite shut 
at I, the steam still left in the cylinder being compressed between 
I and B. The rounded corners, such as BE and FG, show the 
slow acting of the steam valves in closing and opening the steam 
ports, which is called wire drawing. In the ideal engine these 
rounded corners would become sharp ones. 

Determination of I.II.P. from the diagram. —Referring to the 
diagram, Fig. 180, the horizontal distance between the extreme 
points of the diagram, i. e. between the vertical lines ^'yS^and BCj 
represents the stroke of the engine piston in feet. The ordinates 
of the diagram perpendicular to the atmospheric line O'A' repx’e- 
sent to the scale of the indicator spring used the pressures of the 
steam in lbs. per []]". 

If the scale of the indicator spring used in taking the diagram 
= each inch of the ordinates represents 30 lbs. pressure per 
square inch, consequently each square inch of the diagram repre¬ 
sents = 2|ft. lbs. The whole area of the diagram will therefore 
represent the indicated work in ft. lbs. per square inch of engine- 
piston, done on one side of it, during one stroke. Since the 
pressure exerted by the steam on the piston varies at different 
parts of the stroke, we must know the mean effective pressure for 
the complete stroke. This is found by dividing the area of the 
diagram hy the base line, both being reckoned in inch units. The 
result is the value of the mean ordinate of the diagram or mean 
effective Pressure (Pm) square inch of piston area. 

Hence if Z = length of stroke in feet, A = piston area in square 
inches, and N= number of revs, per min. which the engine is 

making, then the I.H.P. = ™ 


33000 


Apparatus. —Generating set to be tested; engine indicator 
(p. 531); speed indicator; planimeter (p. 528); ammeter; volt¬ 
meter; rheostat for absorbing the load from the generator (p. 467), 
and a switch. 

Observations. —(1) Connect the ammeter, rheostat and switch 
in series with one another and with the generator, also the volt¬ 
meter across the terminals of the machine, and open the switch. 

(2) Disconnect the generator and engine and start the latter, 
running alone for some little time before making a test. Prepare 



472 


BLECmiGAL ENGINEERING TESTING 


the engine indicator by first seeing that all the parts are quite 
clean, well oiled, and work practically frictionlessly. Insert the 
right spring in the cylinder suitable for the boiler pressure and 
engine speed to be used, and note its “ scale ” for future reference. 

(3) Blow off steam at the cock which is to carry the indicator 
for a second or two so as to clear away superfluous water and 
dirt. Now screw the indicator to it. Place a card on the drum, 
and make sure the cord which is to actuate the drum will be 
attached to the proper point on the reducing gear so as to give a 
suitable length of diagram for the speed of the engine. 

(4) Turn the cock so as to admit air under the indicator piston, 
cutting off all steam. Then hook on the cord so as to rotate the 
drum and draw the “ atmospheric lineT Then turn the cock so 
as to communicate with the engine cylinder and take a full 
diagram, noting simultaneously the speed of the engine, and in 
addition which end of the cylinder the diagram is taken from. 

(5) Cut off the steam by the cock, unhook the cord, and quickly 
repeat 3 and 4 at the other end of the cylinder of the engine. 
This interchange should be repeated two or three times so that 
an average may be obtained, for the constant speed, when 
working out the results. 

(6) Now run the generator by the engine, absorbing f, and 
full load successively in the rheostat, repeating 3-5 at each 
load for the same speed, both load and speed being maintained 
constant during the time required for taking the readings. Note 
the volts and amperes at each load, and tabulate your observations 
as follows— 


Namk . . . 


Datis ... 


Engine : No.. . . Typo ... Maker . .. Normal I.H.P. =... Speed = ... Pressure=. . 

Dynamo: No.... ... ... „ Volts =... Amps. = ... Speed= , . . 

Scale of Indicator Spring used = . . . Type of Indicator . . . No. = . .. 

Length of Engine Stroke (£) = ... feet. Area of Piston (A) = . . . sq. ins. 


o 

Diagram. 





Condition under wh: 
the test is made. 

End of Cylinder 
from which 
Diagram taken. 

Speed of Engine 
(N) 

Kevs. per min. 

Mean Effective 
Pressure 
lbs. per 

II 

di 

“ S 

o 

f—t 

a 

o 

33000 

to 

-4J 

o 

> 

Amps. A, 

H.P. developed 

746 

Combined EfHcienc 

^ “ IHP' 

1 




































ELECmiGAL ENGINEERING TESTING 


473 


Note. —The value of in the above table is the mean of the 
means of the worked-out results for the two ends of the cylinder 
of a single cylinder engine. In the case of a compound or triple 
expansion engine, the I.H.P. can be found from the diagram taken 
from either cylinder as follows— 

Sum the products of A and for each cylinder {P^ being 
given by the diagram for that cylinder found in the usual way) 
and divide by the A for that cylinder from which the particular 
diagram under consideration was taken. Thus if ^2 = Piston 
area in sq. ins. of, say, the ^^intermediate cylinder^'* of a triple 
expansion engine, the mean effective pressure to be used in the 

formula, say P^^ ~ where '% indicates the sum of the pro- 

ducts for the H.P. intermediate and L.P. cylinders. 

The value of the mean effective pressure as obtained from any 
particular indicator diagram can be obtained by the aid of the 
planimeter, the use of which in measuring the area of the diagram 
is described on p. 530. 

General Observations on Jointing 
Electric Lighting Cables. i 

Good and reliable joints both in core and insulation can only be 
made with practice, care, and attention to the following essential 
details :—All joints in conductors must be as mechanically and 
electrically perfect as possible, for they are in most cases a source 
of weakness in an installation. 

The Joint. —That of the metallic core should have a conductivity 
not less than that of an equal length of the ordinary core, if 
possible, and to obtain this care must be taken not to nick the 
copper cores of the cables to be jointed either with the paring 
knife or pliers, which not only reduces the conductivity, but 
causes the wire of the strand so nicked to break off at once if 
bent at that point. Before making the joint, all the wires must 
be straight and thoroughly cleaned with fine emery cloth, care 
being taken not to remove the tinning of already tinned wires. 
The cleaned wires may preferably be re-tinned with the soldering- 
iron, and must be handled as little as possible, even with clean 
hands. 



474 


ULEGTRIGAL ENGINEERING TESTING 


No joint will ever solder properly unless it is quite clean 
throughout, and in a hot, clean, and well-tinned soldering-iron. 

The Soldering-Irons. —These must be properly grooved to take 
the size of joint to be soldered, and should never be allowed to 
get too hot and “ burn.’^ This always gives rise to an excessive 
lurid green flame, and is not only injurious to the “copper bit,” 
but burns all the tinning off them, thereby giving extra labour 
and wasting time in re-tinning. Irons may be cleaned with either 
sal-ammoniac, emery cloth, or carefully with a suitable file. Salts 
or soldering fluid may be used as a flux in tinning them. Irons 
should be well tinned, and hot enough when used to be unbearable 
when placed about 1from the cheek. They should be wiped 
when taken out of the stove before applying to the joint. Quick 
soldering is essential, as continued application of heat seriously 
weakens copper wire and makes it brittle. Too great a heat 
causes solder to “ rot ” and become useless. 

Solder. —This should be in thin sticks, and should contain 
enough tin to enable one to hear it crinkle when bent double 
close to the ear. 

Flux. —Nothing but resin (applied in the lump) should be used 
in soldering copper joints. All liquid fluxes and other substances 
containing corrosive ingredients should be avoided if the joint is 
required to remain unimpaired with time. 

Insulation. —Great care should be taken to make the insulation 
of the joint as nearly as possible equal to that of the rest of the 
cable. In rubber-insulated cables the braiding or taping is 
removed from the rubber without nicking it, and the pure I.-R. 
strip wound with lap winding over the joint and tapered ends of 
the rubber thus bared. I.-R. solution is now rubbed over the 
joint, but must never touch the bare joint. The taping should be 
done tightly, and be quite solid when completed. 


(i6o) Detailed Instructions for Jointing 
Electric Light Cables. 

Introduction. —For successful and efficient jointing the follow¬ 
ing remarks must be rigidly adhered to— 

(1) In haring any wire or cable preparatory to making a joint, 


ELECTRICAL ENGINEERING TESTING 


475 


great care must be taken not to nick any one or more of the copper 
wires forming the core, which would not only cause the wire so 
nicked to break off on bending it once or twice, but would also 
diminish the sectional area of the cable and so also its current 
carrying capacity. 

(2) In cleaning the copper wires of the core fine emery cloth 
must be used and all dirt removed, but as little as possible (if 
any) of the original tinning. 

(3) Just sufficient and no more cable must be bared as will 
make a satisfactory joint, considerations of the cost of insulating 
materials, and particularly of the ultimate insulation resistance 
of the joint, making it imperative to keep the dimensions of the 
joint, in the matter of length, a minimum. 

(4) Cleanliness is of vital importance in the actual winding or 
making of the joint, and a few extra seconds spent in insuring 
this will almost invariably save many minutes, much solder and 
soldering flux in the end, and even possibly the necessity for a 
second attempt at the whole joint. 

(5) A badly made joint, or a badly insulated one, is a source of 
considerable danger in an electric light installation. 

(6) Too much attention cannot be paid to the soldering irons, 
as it is perfectly hopeless to attempt to solder a joint with—a 
dirty iron, badly tinned iron, or a soldering iron that is not hot 
enough. 


Course in Jointing Electric Light Wires, Cables, and 

Mains. 

The following series of joints constitute a course in the actual 
practice of ^'‘jointing making ” which the author instituted in his 
department at The University, Leeds. They comprise practically 
all the principal distinctive types of joints commonly met with in 
practice, and which might be required to be made by any ordinary 
wireman— 


No. 

1 

2 

3 

4 
6 
6 


Twist-Joint between two 


S.W.G. insulated E.L. wires. 




No. 18 

j, if if No. 14 

X- ,, of a No. 18 on to a No. 14 

X- ,, of a No. 14 on to a No. 7/18 ,, 

Britannia-Joint between two No. 10 S.W.G. bare copper wires. 
Scarf- ,, ,, f, No. 6 




y y 
yy 

9f 


yy 

>1 


yy 

cable. 




yy 


470 


ELECTRICAL ENGINEERING TESTING 


No. 

7 Twist-Joint 

8 T- 

9 Twist- 

10 T- 

11 Twist- 

12 T- 

13 Twist- 

14 T- 

15 Twist- 


between two 


>> 


> 5 


)) 


16 T- 

17 Twist- 

18 T- 

19 Twist- 


99 


> 9 
9 9 


99 


99 


99 


Eo. 

No. 

No. 7/14 
No. 7/14 
No. 19/16 
No. 19/16 
No. 37/16 
No. 37/16 
No. 7/16 


7/18 S.W.G. insulated E.L. cables. 
7/18 


99 


9 9 


99 


9 9 


9 9 


99 

S.W.G. insulated lead-covered 
E.L. cables. 

No. 7/16 S.W.G. insulated lead-covered 
E.L. cables. 

No. 16 S.W.G. insulated gutta-percha- 
covered wires. 

No. 16 S.W.G. insulated gutta-percha- 
covered wires. 

No. 19/16 and a No. 7/18 S.W.G. insulated 
electric light cables. 

20 Slanting between two No. 19/16 S.W.G. insulated electric light 

cables. 

21 Twist-Joint on a large concentric lead-covered electric light main. 


Note.—N o. 5 is a joint used for aerial telegraph and telephone lines. 
17 and 18 are joints used for telegraph work principally. 


Nos. 


Twist-Joints Nos. 1 and 2. 

To prepare. —Carefully bare, with a s/iarp knife, about 11- 
inches of the ends of the two wires to be jointed. This must not 
be done by a cut perpendicular to the wire, but by a short slicing 
motion round the wire, when the piece of insulation will in most 
cases come off whole with a suitable pull. 

Clean each bared wire with fine emery cloth, straighten and 
place them across each other, then lightly gripping them at the 
crossing point with a pair of pliers, bend one free end round the 
other wire. Do this with the other end and finally straighten 
and trim the ends up close, so that they do not project outwards, 
as they are then liable to pierce the insulation. 

To solder. —Place the joint in a well-tinned groove of the iron 
containing solder, then when hot just touch with a lump of resin 
and draw a thin stick of solder over the joint. This usually 
suffices, but if not, repeat the operation, using very little resin. 
The soldered joint must leave the iron quite bright and without 
any globules of solder hanging to the underside of it. The 
soldered joint should appear as in Fig. 181. 

To insulate. —When cool, taper the ends of the insulation, and 





ELECTRICAL ENGINEERING TESTING 


477 


starting from over the rubber of the wire, wind the joint over 
with a spiral half ovei'lap of pure I.R. strip (para tape) to the 
other end, gently stretching the tape all the time so as to obtain 
2ifirm (jiot spongy) layer of I.E,., which, since it is wound in half 



Fig. 181. 


overlap, constitutes a double layer. Apply I.E. solution to the 
outside of this I.E. lapping (on no account next to the copper 
joint) and rub evenly all over the lapping with the finger. 
Next, when the spirit has evaporated out of the rubber solu¬ 
tion, wind on a similar layer of black prepared rubber tape, 
overlapping the outer insulation of the wire at each end, and 
sealing the ends down with solution. Lastly, paint the outside 
of the joint with black waterproof varnish and allow it to dry. 


T-Joints Nos. 3 and 4. 


To prepare.— Carefully bare about 1;^ to inches of the wire 
to be tapped (the larger of the two) and clean withfne emery 



Fig. 182 


cloth. Bare about Ij of the end of the other wire, clean and 
straighten, then placing this across the other wire twist it round 
two or three times to produce the joint shown in Fig. 182, and trim 




























478 


ELEGTRIGAL ENGINEERING TESTING 


the end so that it does not project to any extent. Solder in the 
manner described for joints 1 and 2. 

To insulate. —Proceed as in these last-named joints, but on 
arriving at the T with each serving, carefully branch off down it 
and back, stretching the tapes more tightly to allow for increased 
thickness of insulation; finally, continue along the remaining 
straight portion of the cable, and varnishing over the last layer 
of tape. Considerable care is required in insulating a T-joint, as 
it is more difficult to get round the corners (i. e. angles) of the 
T with the tapes, and these parts therefore are most liable to 
imperfect insulation. 

Britannia-Joint No. 5. 

To prepare. —Gently straighten the ends of the wires to be 
jointed by lightly tapping them with a mallet on the anvil. 
Clean each with fine emery cloth, tin them both for a distance of 
about 2 inches from the end, and bend sharply round the tips of 



Fig. 183. 

their ends. Next place them together with the bent ends point¬ 
ing in opposite directions, and with an overlap of about 2 "; then 
bind them together with about No. 20 tinned copper binding wire 
as shown in Fig. 183. Trim off the ends of this wire, and solder the 
whole into one solid mass as described in joints Nos. 1 and 2, 

Scarf-Joint No. 6. 

To prepare. —Gently straighten the ends of the wires to be 
jointed by lightly tapping them with a mallet on the anvil. 
Clean each for a distance of about 1 inch from their ends with 
emery cloth. 



Fig. 184. 

Next scarf them with a fiat file as shown in Fig. 184, so that 












































































































ELECTRICAL ENGINEERING TESTING 


479 


they taper to thia edges and fit. Then tin them both, wiping off 
nearly all the superfluous solder by a clean cloth. Now warm 
the ends up, and when the surface solder on the scarfed portions 
is melted, place them together to form a continuous wire and 



Fig.185. 


allow to cool. Bind the joint with No. 18 or 20 tinned copper 
wire for about f" from the centre each way as in Fig. 185. 
Lastly, trim the ends of the wire and solder into one solid mass, 
care being taken to just keep the scarfed ends in gentle contact 
while soldering and until set. 


Twist-Joints Nos. 7 and 9. 

To prepare. —Carefully bare about 4" of the two ends to be 
jointed with a sharp knife and a slicing motion (not a cutting one 
perpendicular to the cable). 



Fig. 186. 


Separate out each wire and clean them all with fine emery 
cloth carefully, so as not to remove any tinning if possible. 
Straighten each, cutting off half the centre wire of each cable, 
and then re-twist the outer six up to the end of the centre wire 
with about the same pitch as the rest of the cable itself in both 
cases, arranging them so that the six free straight ends form a 
cone with its apex at the end of the centre wire. 

Now push the cones together, so that the six wires of each 


































































480 


ELEGTRICAL ENGINEERING TESTING 


interlace alternately, and their apexes touch as shown in Fig. 186, 
i. e. the two middle wires butt against each other. Now press 
down the left-hand set on to the cable, and hold tightly in the 
hand. Then wind with the other hand the remaining six wires 
of the other cable, one hy one, by, say, half a turn at a time, and 
evenly to, say, 1 or 1;| inches from the centre, snipping off each 
what is not wanted, and trimming the ends so as not to project 
outwards. Repeat these operations with the other half, and 



Fig. 187. 


finally trim the centre also by pressing with the pliers, but not 
scraping the cable in so doing. 

N.B.—The joint may then appear like Fig. 187 or 188, preferably 
the latter, which makes the neater joint and is, as seen, wound 
in the same sense as the main cable. 

To solder. —Place in the well-tinned groove of a fairly large 
soldering iron and run in some solder around it, then when quite 
hot just touch the joint for merely an instant with a lump of 



^ Fig. 188. 


resin, and draw a stick of solder over the joint. This, as a rule, 
will suffice to cause the whole joint to become tinned ; if it does 
not, repeat. When properly tinned the joint should leave the 
iron in a bright state, and with no globules of solder hanging to 
the underside of it, and should be one solid mass. 

To insulate. —^When cool taper the ends of the insulation, and 
starting over the rubber of the cable, wind on spirally with a half 
overlap two layers of pure I.R. tape in opposite directions and 
with enough tension to make the insulation firm and solid. The 
end of this tape is fixed down by I.R. solution, which is also 





























ELECTRICAL ENGINEERING TESTING 481 

applied to the outside of the rubber-taping all over the joint by 
means of the finger. When the spirit has evaporated repeat 
the above winding process with two layers of black prepared 
rubber tape overlapping the outer braiding of the cable. Lastly, 

^ arnish the joint all over with black waterproof varnish and 
allow it to dry. 


T- Joints Nos. 8 and 10. 

To prepare. Carefully bare about 2^' to 3 ^^ of the cable to 
be tapped. Clean the outside of the stranded core with fine 
emery cloth and re-tin well. 



Fig. 189. 


Next carefully bare about 3" of the end of the other cable, 
separate out all the wires, clean each with fine emery cloth, 
straighten and re-twist them up to about from the end of the 
insulation in a slightly sharper twist than the ordinary cable. 
Now spread the 7 wires out to form a V, the apex of which is 
at from the insulation, 4 wires being one side and 3 the other. 

Next press the other cable into the V as shown in Fig. 189, 
then holding the two tightly together, wind one hy one the 4 
wires round in one direction, and the 3 in the opposite direction 
to, say, I" either side of the centre. Clip off what is not wanted 

11 









482 ELECTBIGAL ENGINEERING TESTING 

of each wire, and trim so as to not project outwards, when the 
joint should be as in Fig. 190. 

To solder. —Repeat the operation described for joints 7 and 9. 
To insulate. —Repeat the operation described for joints 7 



and 9, except that when the T is reached, wrap carefully round 
the angles, down the T and back, stretching the tapes tighter 
here to allow for the extra lapping this part will receive, then 
finish off the joint as in 7 and 9 above. 

Twist-Joint ISTo. 11. 

To prepare. —Carefully bare about 4J" of the two ends to be 
jointed, and separate out the outer layer of 12 wires, clean them 
with fine emery cloth and straighten. Without unwinding the 
inner 7 solder them into a solid mass and cut half off in both 
cables alike. Row re-twist the outer 12 up to the end of the 
inner 7 with about the same pitch as the ordinary cable, and 
arrange them to form a cone with the apex at the end of the 
inner 7. Then pushing the two cones together and interlacing 
alternately, proceed to finish exactly as in joints 7 and 9. 

T-Joint Ro. 12. 

To prepare. —Carefully bare about 3" of the cable to be tapped, 
clean the outside of the stranded core with fine emery cloth, and 
re-tin it well. 












ELEGTEIGAL ENGINEERING TESTING 


483 


Next carefully bare about 3^" of the end of the other cable. 
Separate out each wire, clean with fine emery cloth, and straighten. 
Next re-twist both inner and outer sets up to about from the 
insulation with a slightly sharper pitch than the ordinary cable, 
then carefully spread out all the wires in the best possible manner 
to form a V with 10 one side and 9 the other. Lastly, press the 
other cable into this V, and finish tho loint precisely as in Nos. 
8 and 14. 

Twist-Joint No. 13. 

To prepare. —Carefully bare about 5J"of the ends of the cables 
to be jointed, unwinding the outermost layer of 18 wires, and 
proceed precisely as in No. 11, except that half the inner 19 
must now be cut off after soldering them together, 

T-Joint No. 14. 

To prepare. —Carefully bare about 4J" to 5" of the cable to be 
tapped and about 5 of the end of the other, and proceed exactly 
as set forth for joint 12, excepting that the V will now have 19 
wires on one side and 18 the other. Finish it off as there 
indicated. 

Twist-Joint No. 15. 

To prepare. —Carefully cut the lead sheathing away, without 
in any way nicking the copper core of the cable, for about 4" of 
the ends of the cables to be jointed. Next slip on to one cable, 
to some little distance from the joint to be made so as to be out 
of the way, a lead sleeve consisting of a length of lead piping, a 
little larger than the size of the lead-covered cable and some 2" 
longer than the finished joint will be. Then proceed to make the 
joint and insulate it precisely as in Nos. 7 and 9, taking extra 
care to get the insulating tapes on tightly and efficiently. Now 
slip back the loose sleeve over the joint and either carefully 
“ solder ” or “ solder-wipe the ends, thus completely sealing in 
the cable. 

Uote.—If lead piping to the right size is not available, a sleeve 
may be cut out of lead sheet, bent round the joint and finally 
sealed along the edge; this, however, does not make so neat a 
joint as that with the pipe. 


484 


ELEGTRIGAL ENGINEERING TESTING 


T-Joint jSTo. 16. 

To prepare. —Carefully cut away about 4" of the lead sheath¬ 
ing of the cable to be tapped, great pains being taken to avoid 
nicking the copper core. Next remove about of the lead 
sheathing from the end of the other cable and slip over this 
latter a short sleeve of lead piping slightly larger than the 
ordinary lead-covered cable, and of sufficient length to cover the 
insulation of the joint and overlap the end of it. Now make and 
insulate the joint precisely as described in Nos. 8 and 10 and slip 
back the small sleeve over the insulation; also cut out a piece of 
sheet lead to form a sleeve over the rest of the joint, its ends 
overlapping those of the lead covering on the cable by about 1 "; 
lastly, train and trim these lead coverings to fit closely, and solder 
or solder-wipe the seams to make a neat water-tight joint. 

Twist-Joint No. 17. 

Prepare and make the joint precisely as in Nos. 1 and 2, when 
it will have the appearance shown in Fig. 191, and it may prefer¬ 
ably be kept as short as possible in order to facilitate insulating it. 

To insulate.—Warm up the G.P. covering on either side of the 
joint, then work and draw down with moistened fingers that on 
one side half-way over the joint as in Fig. 192, and next that on 
the other side, giving the form shown in Fig. 193; work the two 
draw-downs together and sear all the joint with a hot searing 
iron. 

Wrap a strip of G.P. well warmed over a flame round the centre 
of the draw-down as in Figs. 194 and 195; now work this roll 
both ways with moistened fingers until it is uniformly distributed 
over the joint as seen in Fig. 196, finally smoothing all over with 
a searing iron, and lastly with wotted fingers so as to leave the 
whole joint quite smooth. 

T-Joint No. 18. 

Prepare and make the joint precisely as in Nos. 3 and 4, 
keeping its dimensions small. 

Insulate in a somewhat similar manner as in No. 17, working 
the three branch G.P. coverings into each other at the T. Warm 


ELECmiGAL ENGINEERING TESTING 


485 


narrow G.P. strip must now be wrapped round the T, first one 
way and then the other, and drawn down in the three directions 
so as to leave a clean, smooth, insulated joint. 








Twist-Joint No. 19. 

This joint is made in precisely the same way as No. 11, and is 
insulated in the same manner, and is shown in Fig. 197. 





































48G 


ELECTRICAL ENGINEERING TESTING 


Slanting T-Joint No. 20. 

The cable to be tapped is prepared exactly as in No. 12, the 
other cable having its inner seven wires soldered together, cut 
half off and scarfed to the desired angle or slant. The remaining 
twelve wires are then wound as before in opposite directions, six 
one way and six the other, round the other cable, giving the joint 
shown in Fig. 198. The scarf should butt up against the tapped 
cable and be soldered to it in the final sweating. 



Fig. 197 . 


Straight Concentric-Joint No. 21. 

We will assume that the cable or main is a 37-stranded, lead- 
covered and armoured one, the gauge being any one of the usual 
sizes in practice. Treat each of the two ends to be jointed as 
follows :—Unwrap the outer serving of yarn or hemp for a 
distance of something like 13 inches from the end, but do not cut 
it off. Next unwrap the strip armouring for about the same, or 
a slightly less distance, say 11 or 12 inches, without cutting it off. 
Then unwrap the inner serving of yarn, which separates the 
armour and lead sheathing, for some 10 inches, without cutting it 
off. 

Now remove the lead sheathing for about 9 inches altogether. 

Joint in Inner Main. —Carefully bare the outer conductors 
for about 7 inches from the end with a sharp knife. Spread 
out and clean them each with fine emery cloth and straighten, 
leaving them outspread. Next bare, carefully, the inner 
conductors for about 5 inches. Spread out and clean each 
conductor with fine emery cloth and straighten each. Ke-twist 
up the innermost 19 as they were originally, and cut J of this 
inner 19 strand off. Remove any jagged edges and solder so as 
to form them into a solid mass. 

Each main will now appear as in Fig. 199, except that the 
unwrapped ends of the yarn and armouring are not shown. 
Now cut off f of every alternate wire immediately surrounding 








ELECTRICAL ENGINEERING TESTING 


487 


the inner 19, and bringing the two ends of the cables thus 
prepared together, so that the inner 19’s butt up to each other. 
Interlace these outer wires so that the respective pairs also butt 
though alternately on either side of the centre. Then bind this 



Fig. 198. 


first joint of the inner cable with four strips of tinned copper 
binding wire, each of some ten turns, and the strips equally 
spaced over the joint, so that the three sets of butts come in 
between the four strips of binding wire. Lastly, by means of two 



Fig.199. 


ladles, keep pouring melted solder over the joint, catching it in 
"Vhe ladle held underneath and using it over again and again 
until the joint taJces the solder and becomes tinned and soldered 
into a solid mass. Fine powdered resin must be used as a flux in 
just sufficient quantity. 






















































488 


BLEGTRIGAL ENGINEERING TESTING 


When cool enough, taper the end of the first insulating cover¬ 
ing, when the joint should then present the form shown in Fig. 
200. Now wrap on tightly in the usual way, first pure rubber 
strip, with the application of rubber solution between layers, and 
then prepared tape up to a thickness slightly exceeding the 
other insulation. 

Joint in Outer Main. —Wrap a sheet or sleeve of copper plate 
which has been previously cleaned on the outside carefully with 
emery cloth, and which is of such a thickness as to be comfortably 
pliable. This sleeve must be the length of the outer conductors, 
and make one turn or wrap fitting the insulated cable closely. 
Now cut off half of every alternate wire of each outer main and 
interlace, after cleaning and straightening each as before. Then 
bend them closely over the sleeve of copper by tinned binding 
wire in, say, four strips about wide, the butts of the conductors 
being between the pair of strips, either end. 

Solder as before with the ladles. The joint now has the 
appearance shown in Fig. 201. 

When cool enough insulate up in the usual way, tapering the 
ends of the old insulation of the cable beforehand. 

Protections to insulated joint. —Vfrap a piece of sheet lead 
over the last insulation, so as to form a sleeve of just one 
complete turn and overlapping the ends of the ordinary lead 
sheathing of the cable. Then solder icipe the ends and down the 
seam so as to make a good water- and air-tight joint. ' 

Note. —Any part of the lead may be previously painted if 
desired so as to prevent the lead solder wiping from taking to 
that part. Next re-serve the inner yarn as far as it will go over 
the lead joint, adding more to complete the serving. 

Then repeat this operation with the strip armouring, and lastly 
with the outer yarn, when the joint may finally be well tarred 
over and is then comjAete for laying in. 

Generally speaking, all joints larger than about can be 
more expeditiously and effectively soldered by using flux and 
pouring molten solder out of one ladle over the joint and 
catching it in a second and larger ladle underneath. The reason 
is that by this means the joint can be raised to, and kept at, the 

proper temperature, which is difficult to obtain with soldering 
irons. 







































































































































































































































































































APPENDIX 


PROOFS OF FORMULAE 


Deviation of the deflections of Reflecting 
Galvanometers from the direct propor¬ 
tional law. 

It has been stated on p. 6 that the scale deflections of a 
D’Arsonval galvanometer are directly pro 20 ortional to the currents, 
producing them. Though this is not rigorously correct, it is 
sufficiently true for most practical purposes in the usual forms of 
instruments belonging to this class. For very accurate work, 
however, it is necessary to apply a correction, usually amounting 
to a small fraction of 1% for deviation from this law, and this 

Let 0 be the zero of the scale 
PQj presumably in the centre, 
though, for convenience, only one- 
half of the scale is shown, and 
let Od and OD be the scale de¬ 
flections of the spot of light on 
PQ from zero for currents and: 
G 2 through the galvanometer 
coil. 

If B is the centre of the needle 
ns, then OB is the incident ray 
of light from some source at 0 
and Bd, BD the reflected rays for 
the two positions of the mirror and its attached needle ns. 

By drawing the normals to the mirror in each position we get 

490 


we now proceed to indicate. 


Q 


fk 

d 

a 

0 










s;s 


Fig. 202. 







ELECTRIGAL ENGINEERING TESTING 


491 


Ra and RA respectively, and it can at once be shown that the 
angle dRR = ^ciBAj or that the angular motion of the mirror is 
half that of the reflected ray. 

Now since ns is contained by the plane of its coil for no current 
and is parallel to PQ, we shall always have (for the small angular 
motion of 7is usually obtained in mirror galvanometers)— 

= i ORd : tan. J ORR, 


= tan. J 


Od 

OR 


tan. 


jL OR. 
2 OR. 



Od 

OR 



(accurately), 


or if Od and OR are not very different and are small, then— 
Cl : G^^Od : OR (approximately). 


Measurement of the Internal Resistance of 
Secondary Cells. (Fall of Potential 
Method.) 


Proof of Formula. —Eefening to p. 76, let ^=the total 
E.M.F. of the cell or battery, and V the potential difference at its 
terminals, when sending a current 

If then B is the internal resistance of the cell and R the 
resistance of the external circuit, we have by Ohm’s Law—• 

Fall of Potential round external circuit = AR = F, 
and „ „ in the cell itself =AR. 

Hence we must have E = AR + AR = A[R + R). 

But the Fall of Potential in the cell itself is also = E—V, 


and 


» • 


E-V^AR, 


R = 


E-V 

A 


ohms; 


or thus 


E_A(R + R) 
V AR 


Hence 




but AR = V, or R = 


F 

A 



E-V 


ohms. 


A 














492 


ELEGTBIGAL ENGINEERING TESTING 


Measurement of Resistance. (Wheatstone 

Bridge Method.) 

Proof of Formula. —Eeferring to Fig. 33 I., p. 82, let Fp 
and Fg = the Potentials of the points P, N and T respectively; 
then the point Q will also be at the potential since when the 
bridge is “ balanced ” no deflection will occur on the galvano¬ 
meter, owing to there being oio difference of potential between 
N and Q to cause a current to flow. Now let and G^ 

be the currents passing through the resistances ^ 2 , and 
respectively of the arms of the bridge. 

Then “ ^2 = = ^ 4 ^ 4 » 

and V^-V^ = G^r^=^G^r^] 

but since on balance being obtained no current flows through the 
galvanometer, ^. e. between the points iFand Q, we get 

G^ = G^ and G^ = C^g, 


whence by division 

_ ^ 4 ’ 4 
C’lri 

or 



^2 ^*1 

and 

r^r.^ = r^r^. 


The resistance in the galvanometer and battery circuits is 
immaterial, so long as it is nob great enough to diminish the 
sensitiveness of the tests; consequently it may vary without 
vitiating the results at all. 


Measurement of Low Resistance. (Potential 

Difference Method.) 

Solution of Inferences. —Eef erring to Fig. 34, p. 85, let 
A = current flowing through the resistances r and R in series, 
and V, V= Potential Differences across their extremities re¬ 
spectively. 

Then from Ohm’s Law we have 


but since the galvanometer resistance is high compared with 









ELECTRICAL ENGINEERING TESTING 


493 


either r or Rj we have its deflections and dji across these re¬ 
sistances respectively proportional to v and V, whence 

dji _ dj, 

~R~r 

and /. the unknown resistance, assumed to be (r), is 

r = ^ R ohms. 

dji 

Assumptions. — (1) That no fluctuations of the current A have 
occurred in the interval of time between the observations dji and 
d^ of any particular pair. 

(2) That the introduction of the galvanometer across the re¬ 
sistance has not altered the P.D. at their respective terminals. 

(3) That the deflections are proportional to the currents, which 
is very nearly true for ordinary reflecting instruments. p 

Errors may arise from the warming up of the resistances to be 
compared, due to the passage of the current, and the consequent 
alteration of resistance. The current should not be strong enough 
to do this. 

Also from the presence of thermo currents caused by the 
warming up of a junction of two dissimilar metals. 

Lastly, from the inconstancy of the current, which can be 
minimized by taking readings with one of the resistances before 
and after that with the other, and noting the mean of the two. 


Measurement of very high or Insulation 
Resistance. (Direct Deflection Method.) 

Solution of Inferences. —Keferring to p. 101— 

Let b = the internal resistance of the battery, 
g = the galvanometer resistance, 

(7^ = the currents through the galvanometer, causing deflec¬ 

tions d^ dr, when the resistances R and r are in circuit respectively 
and K be a constant converting these deflections to actual 
currents. 

Then if E is the E.M.F, of the battery we have by Ohm’s Law 

^ Er E ErE _ 

— ^ = (ft + 6) {S!i+g) + gS^s 

A + 6 + o— 






494 


ELEGTRIGAL ENGINEERING TESTING 


S. 


E 


SrE 


Similarly (7, = Kd, = ^ x — ^ = (r + 6) {S,.^g)+gSr 

whence by division 

dn _ {r + h) {S^ + g) + gSr ^ ^ 

dj. {R+ b){SR +g)+gSxi Sr 

Now h will always be negligibly small compared with R ov r. 


Hence 


dn r {Sr-\‘g)-\-gSr 

1 / /'tf \ /~v a 

a. 




or 


d 


R 


'R{SR+ff)+9SK Sr 


( 1 ) 


When no shunts are used Sr and Sr both = infinity and 

dn {R+g) = dr{r + g) 

Lastly, if (g) is negligibly small compared with R and r, then 

cZ jj R “ dr 


and 


dR 

r = ^ R 

dy. 


( 2 ) 


The assumption made in both formulas 1 and 2 above are 
that— 


(a) The E.M.F. of the testing battery remains constant 
throughout the tests, which is justifiable owing to its working 
through such high resistances. 

(5) The internal resistance of the battery is so small compared 
with R and r as to be quite negligible. 

In the relation (2) above, it is assumed that— 

(c) No shunts are used at all with the galvanometer. 

{d) The galvanometer resistance (y) is so small compared with 
R and r as to be quite negligible. 


Insulation Resistance of Electric Light 
Installations while working. 

Solution of Inferences. —Let A and B be the two points of the 
circuit most convenient for attaching the wires to, that come from 
the two-way key K. Let 7'^ = I’^sistance of the voltmeter, and 
R ^2 ~ insulation resistances of the + and - sides, respectively, 














ELECTRICAL ENGINEERING TESTING 


495 


of tlie network or installation, in ohms. Then that of the whole 
system, everything included, is ^ 

Rl d" L 2 

In Fig. 203, E represents the earth or nearest gas or water-pipe. 

If then Fj and Fg are the voltmeter readings when placed 
between the -fand — sides of the circuit and earth, and F 
the voltage between the mains, we have 

+ P 



k- 

B ^ ( 

? 

s 

J 

U- 


7 



R, 


R. 


Earth 


Fia. 203. 


Fall of P.D. between A and viz. Fj = 

IXy 


Ri + r. 


+ Rn 


Rq ^'v 

III d" ’’v 


Similarly the fall of P.D. between B and E^ viz. F- p — 


Ro d- r,, 1 


Since ERA and EVA are in parallel, and the combination in 
series with EQB. 

... V^+V^^V-. - 


+ li^ 7‘y + Hy 

_1_1 / in 

■ Vy+V^~ VV’^'jiy + lilrJ 


or 


also 


R = ?•« 


1 


i?2 = .R 


Vy+V, rJF-(r. + F ,) 1 
F “ F 

liy = rJv-{Vy+V,)] 


- 

{V-^A-V^ representing the absolute sum of the two voltages, 
assuming they are to opposite sides of zero. 



























49C 


ELEGTEICAL ENGINEERING TESTING 


Insulation Resistance of a Storage Battery. 

Solution of Inferences. —Eeferring to Fig. 50, p. 1^ — 

Let Pj Pg ^3 • • • I*® points on the battery which are partially 

earthed, 

. . . be the resistances of these earths, 

G-^C^C^ . . . be the currents flowing from P^ Pg and P 3 .. • 

to earth when G is connected, K closed, 
and r = 0 . 


E.M.F.s between P and P^PgPg . . . 
respectively. 

Then if all the currents flow from battery to earth 

Gi Pi - Gog = Ei. G^g- G^R^ = E^, 

Gc^R'2, — GQg = E^^ Gog — G^Rq = Ei^, 

etc. etc. 


Also 
Hence—■ 

E-^A-Gog , E^I-G^g 
R 


+ 


(7i + Pg d" P 3 + . • . +Pg“0 . 

Gqg — E^ Gi^g — E 


+ ... + 




^ +. . . 4- (7g = 0. 


■ ^4 ■ ••• - 74 It, .. 

Now when Gq falls to P,. by increasing g to (^ + ?*) we have 
Et + Cr{g + r) E, + C,.{g + r) Cr{g + r)-E^ 

A’l ^ K, +•••+ It, 


+ 


Gr {g + r)-E^ 
Pn 


+ . . . + Gr ~ 0. 


Hence from the difference of these two equations we have 

{Cag-Gr{g + r)} + • • ) + C'<,-C.=0, 

2 


whence 


T:,_ GGg-Gr{g + r) 

n n 
\jr — L/ 0 


Insulation Resistance of Dynamos and 

Motors. 

Solution of Inferences. —The proof of the formula employed in 
this test is precisely the same as that given on p. 494 for the 
insulation resistance of electric light installations. It will not 
therefore be repeated here. 












ELEGTRIOAL ENGINEERING TESTING 497 


Efficiency of Direct-Current Generators. 
(Hopkinson’s Electrical Method.) 

Solution of Inferences. —Eeferring to Fig. 85, p. 224, let 
Fj V 2 Fg be the terminal voltages of a, (I and y respectively, and 
A the main current through them all. Then we have— 

Power developed by the generator a = A Watts. 




>> 


supplied by the auxiliary y =AVq 

given to the motor /3 —^^2 


’y 


Hence 4F3 = ^F 2 -^Fi =--'i (1^2“ ^i)» 

which is the power lost in both machines together. 

If F= normal working pressure of a and j8 when running as 
dynamos, ra and = normal resistances of their shunts. 
y2 Y2 

Then — and — = Watts wasted in their respective shunt 

circuits, neglecting any extra resistance such as and^g* 
let the total internal loss in a = that in (I. Then— 

Efl&ciency of dynamo a 

useful power developed _ 4 Fj 

total power put in ^ a . a tt . F^ 


5 = 


J4Fg + 4F,+ 


V 

• /I 


Efficiency of motor p 


2j3 = 


useful B.II.P. developed _ ^3 + ^ ^ 2 + 

total E.H.P. put in 


72 


'/3 


j/2 

^ r/3 


72 

4 F 2 + — 

2 


F2 




r/3 


If now we neglect the shunt losses — and 

a 

with the outputs of a and /?, then 

2J,_ ., n 

I'l + I 

y, 

y-: 


in comparison 


■s; = 


and Si3= -V{; 


- ■ - 


K K 








498 


BLEGTRIOAL ENGINEERING TESTING 


Lastly, if we assume that the machines are so alike that 

= = (say) 

Then ^ or 5 = \/ ^ 

which is therefore the efficiency of either machine. 


Self-Induction by the Impedence Method 
using Single-Phase Alternating Currents. 


Solution of Inferences. —Keferring to Fig. 132,p. 360, let F„ 
and Vj) be the readings of the voltmeter when an alternating 
and direct current of the same strength A is successively passed 
through the self-induction whose value L is to be determined. 

Let = frequency of the alternating current in _per sec,, 

then its angular velocity 'p = 27^7^. 

If if = the ohmic resistance of the self-inductive circuit we 
have for the direct current, by Ohm's Law, 

R = ohms, 


and for the alternating current we have 

V. 


— = \/ IP-p^ 4- BP' the impedence of the circuit. 
A 


Hence by substitution 


5-v, 



Self-Inductions in Series and Parallel Laws 

of Combination. 

Proof of Formula. —Suppose that we first take the arrangement 
represented in Fig. 135 y, p. 367, in which all the self-inductions 
Lj Zg and are in parallel. 








ELEGTRIOAL ENOINEERING TESTING 


499 


Let them possess ohmic resistance Rq and R^ respectively, 
but no mutual induction. 

i Also let and R^ be the combined or equivalent self-induction 
and ohmic resistance of the combined parallel circuit such that if 
substituted for the parallel branches the same potential difference 
V would exist at the terminals, causing the same current A to 
flow through. 

If then the angular velocity of the alternating current be 
denoted as usual, by /> = where n is the periodicity, we can 
apply the solution obtained by Lord Rayleigh for the impedence 
of parallel circuits and given in a paper by him “ On Forced 
Harmonic Oscillations of Various Periods.”— Phil. Mag.y May 
1886. 

and R^ — 


Thus 

where 

Hence 




A = ^ 


R 


A^ + ^‘2p2 


-I- R^, 




Substituting the values of A and B given above we get 


-V; 


f 

0 


(Z V + i?2)5 




R 


(zy+ Rf, 


1 




Next take the arrangement shown in Fig. 135/?, in which the 
inductions are connected up, two in series and two in parallel. 

Here, since self-inductions in series sum up like resistance in 
series, we have, applying the last equation for two parallel 
branches, that 

V 
A 


— yJ L^p‘^ + R^ 




4- R^ , Zg -f R^ 




L i + L^ ^ L). + L 


(h 

V(A 


+i,f {i,+h)h 
(/,+/,) (4+/J 


p‘ 


^ (Z'j + Xq 4* 4 4 (^] 4 ^2 ^8 

























600 


ELECTBIOAL ENGINEERING TESTING 


+ L,)y + (E, + R,Y V (4 +A)y (^ 3 + ^4? 

y/{^LyY+{^RY 

which is the combined or effective or equivalent impedence of 
this particular branched circuit. 


Electrostatic Capacity of Electrical Cables. 
(Multicellular Voltmeter Method.) 

Proof of Formula —Let 6^^ = capacity of the standard air 

condenser ; — capacity of the cable, 

and /i j/fg = capacities of the voltmeter 
at the potentials Fj and V respectively. 

Then if Ga is first charged in parallel 
with F to a potential F^, the quantity 
of electricity in the charge is (2^ = Fj 
{Ca, + KY)i since the capacities and 
^1 are in parallel. 

If now K is closed so as to put Gj, 
in parallel with the first two and the 
potential falls in consequence to F^, 
the quantity of the charge is still the 
same assuming no leakage, and we have 
Qi-^V^{G, + Ci, + IQ. 

Hence (C„ + 1Q=V^{G, + Cj, + K,), 

V, Cg + K^ 

G, + C^ + K^ 

and by a well-known rule in proportion we have 



Fia. 201. 


or 


^1-^2 


Ga + Ki 


Ca^Cj^ + K.-G.-K-G, 


Og + J^l 

+ Kl 


= V,G, - V^Ga + V,K, - 
Hence V^G^ + V^iq = Ga (F^ - V^) -h 

and ... = 

or neglecting the capacity of the voltmeter in comparison with 


Ga and Gj, we have G^ — Ga 


V„ 





























ELEGTEIGAL ENGINEERING TESTING 


501 


Capacity of Concentric Cables. (Standard 
Magneto Inductor Method.) 


Proof of Formula.— Referring to the test, p. 377, let Nhe the 
number of turns of wire on the standard inductor and F the total 
number of lines of magnetic force threading the gap, then the 
interlinking of turns and field = FN. 

If now = the total resistance in ohms of the inductor circuit, 
the whole quantity of electricity producing the throw dj is 

NF 

Q-^ccdicc — 


Again, let (7 = the capacity of the cable in microfarads, r=the 
potential difference in volts to which it is charged, then the 
(piality of electricity causing the throw (7^ is 

Q2^ d az GVf 


and 1 microcoulomb = 
also 1 ohm 
Hence 


10 


-1 


10« 


ch 

d. 


= 10“^ C.G.S. units of quantity, 

= 10^ C.G.S. units of resistance. 
FN FN 


and 




10-7 X 10» live 
FN do 


100 RVG 
microfarads. 


100 RV di 

For the greatest accuracy we ought to have 

di — d(,. 


Three Voltmeter Method of Measuring 
the‘‘True Electrical Power''in Single- 
Phase Alternating Current Circuits. 

Solution of Inferences. —Referring to Fig. 140, p. 382, lot 
the J mean square value of the voltages, as given by either 
a hot wire or electrostatic voltmeter, be denoted by F, Fj and Fg 
for the positions shown in the figure; also let (r) = the value of 
the non-inductive resistance QR in ohms. 

Then we require the value of tho mean or True Power in 
Watts IF given to the whole circuit PR in which the whole power 
is absorbed; let -u, and Vg be the instantaneous values of the 








502 


ELECTRICAL ENGINEERING TESTING 


voltages corresponding to the ij mean square values V,V^ and Fg 
respectively at any instant («), then v = t\ + V2; also if (a) = 
instantaneous current in amps, flowing through FR at this same 
instant we have the instantaneous value of the power in Watts 
(w) given to PR at that instant as = va; 

but we also have ® since QR is non-inductive, 


Vo Wo 

whence w = va = v — = —— 

r T 

or w^^wr\ 

now since v^v-^ + v^ 

V-^ = V — V2 

and ~ '*^ 2 ) “ ^^ 2 ^* 

Hence by substitution ^v^v^ = ‘^wr — ‘I.v^ ; 
but = v-^ -f + v^ 

^2 _ ^^2 ^ 2wr — 2v^ -f v^ 


Hence 



^2 _ ^^2 ^ ^^2 


\ 

r 


by squaring, 
by substitution. 


Integrating this equation for the period T we get 


and finally 




Efficiency of Transformers. (Blakesley’s 
Three Dynamometer Method.) 

Solution of Inferences. —Referring to p. 423, on which will 
be found a formula derived by Mr. Blakesley for expressing the 
power given to the primary of a transformer in his method of 
measuring the efficiency of such an appliance, Professor Ayrton 
and Mr. Taylor have deduced the following general proof of this 
relation, which makes no assumption whatever as to the nature 
of the current, whether sinusoidal or otherwise, but^ only that 
there is no magnetic leakage between primary and secondary 
windings. This is approximately true for “closed circuit” though 
not for “open circuit” transformers, so that the method cannot 
be considered a very good one. 




ELECTRICAL ENGINEERING TESTING 


503 


Let = instantaneous values of the currents and those 
of the E.M.F.s in the primary and secondary windings having 
iV 2 respectively, and 7? = the mean density of lines in the 
core, then if = ohmic resistances of the primary and secondary 


we have 
But 


■*'108 , n - 


clB 


~ sii^oe we assume no leakage, 


N^ 


, . ^1 — R^Cb-^ + 7 ^ 2^2 > 


multiplying all through by we get 

N 

v-^a-^ = R-^a^ + R^a^ay 

Integrating this last equation between the limits of the period 


T we get 


1 7? N N r 

2 ’Jo = “!**■*■ if 2^7 V 2 *- 

2 

Ilenco if A is the split dynamometer reading we finally have 






APPAEATUS 


Preparation of the Clark Standard Cell. 

Definition of the Cell. —The cell consists of zinc and mercury 
in a saturated solution of zinc sulphate and mercurous sulphate in 
water, prepared with mercurous sulphate in excess, and is con¬ 
veniently contained in a cylindrical glass vessel. 

Preparation of Materials. — The Mercury. —To secure purity 
it should be first treated with acid in the usual way, and sub¬ 
sequently distilled in vacuum. 

The Zinc. —Take a portion of a rod of pure zinc, and solder to 
one end a piece of copper wire. Clean the whole with glass 
paper, carefully removing any loose pieces of zinc. Just before 
making up the cell, dip the zinc into dilute sulphuric acid, wash 
with distilled water, and dry with a clean cloth or filter paper. 

The Zinc Sulj)hate Solution. —Prepare a saturated solution of 
pure (re-crystallized) zinc sulphate by mixing in a flask distilled 
water with nearly twice its weight of crystals of pure zinc 
sulphate, and adding a little zinc carbonate, in the proportion of 
about 2 per cent, by weight of zinc sulphate crystals, to neutralize 
any free acid. The whole of the crystals should be dissolved with 
the aid of gentle heat, i. e. not greater than 30° C., and the solu¬ 
tion filtered while still warm into a stock bottle. Crystals should 
form as it cools. 

The Mercurous Sulphate. —Take mercurous sulphate sold as 
pure, which is white, and wash it thoroughly with cold distilled 
water by agitation in a flask; drain off the water, and repeat the 
process at least twice, but after the last washing, drain off as much 

504 



ELECTRICAL ENGINEERING TESTING 


505 


water as possible. Mix the washed sulphate, in the proportion of 
about 12 per cent, by weight of ZnSO^, crystals, with the zinc 
sulphate solution, adding sufficient crystals of zinc sulphate from 
the stock bottle to ensure saturation, and a small quantity of pure 
mercury. Shake them well up together to form a paste of the 
consistency of cream. Heat the paste sufficiently to dissolve the 
crystals, but not above 30° C. Keep the paste for one hour at 
this temperature, agitating it from time to time, and then allow 
it to cool. 

Crystals of zinc sulphate should then be distinctly visible 
tliroughout the mass. If this is not the case, add more crystals 
from the stock bottle, and repeat the process. This method 
ensures the formation of a saturated solution of zinc and mercu¬ 
rous sulphates in water. The presence of the free mercury 
throughout the paste preserves the basicity of the salt, and is of 
the utmost importance. Contact is made with the mercury by 
means of a platinum wire about No. 22 B.W.G., which is pre¬ 
vented from making contact with the other materials of the cell 
by being sealed into a glass tube, the ends of the wire projecting 
beyond those of the tube. One end forms the terminal; the other 
end, and part of the glass tube, dip into the mercury. 

To set up the Cell.—The cell may be conveniently set iq) in a 
small test tube of about 2 cms. in diameter and 6 or 7 cms. deep. 

Place the mercury in the bottom of this tube, filling it to a 
depth of, say, 1*5 cms. 

Cut a cork about 0*5 cm. thick to fit the tube. At one side 
of the cork bore a hole through which the zinc rod can pass 
tightly; at the other side bore another hole for the glass tube 
which covers the platinum. At the edge of the cork cut a nick 
through which the air can pass when the cork is pushed into the 
tube. 

Pass the zinc rod about 1 cm. through the cork. Carefully 
clean the glass tube and platinum wire, then heat the exposed 
end of the wire red hot, and insert it in the mercury in the test 
tube, taking care that the whole of the exposed platinum is 
covered. 

Shake up the paste, and introduce it without contact with the 
upper part of the sides of the test tube, filling the tube above the 
mercury to a depth of rather more than 2 cms. 


500 


ELEGTBIGAL ENGINEEBING TESTING 


Now insert the cork and zinc rod, allowing the glass tube to 
pass through the hole in the cork made for it. 

Push the cork gently down until its lower surface is nearly 
in contact with the liquid. The air will thus be nearly all ex¬ 
pelled, and the cell should be left in this condition for at least 
twenty-four hours before sealing, which should be done in the 
following way— 

Melt some marine glue until it is fluid enough to pour by its 
own weight into the test tube above the cork, using enough to 
cover completely the zinc and soldering. The glass tube should 
project above the top of the marine glue. 

The cell thus set up may be mounted in any desirable way; 
do it so that the cell is immersed in a water-bath up to the level, 
say, of the upper surface of the cork. Its temperature can then 
be determined more accurately than is possible when the cell is 
in air. 


Instruments for Standard Measurements of 
the Highest Accuracy. 

The potentiometer in general must rank as one of the first 
of this kind, not solely from the point of view of accuracy, but 
also because of the ease and rapidity with which the measure¬ 
ments possible with it can be taken. The principle underlying 
its use is contained in the Clark-Poggendorff method of com¬ 
paring E.M.F.s, and an elementary application of this principle 
in Poggendorff’s method of calibrating a voltmeter, a detailed 
description of which will be found in the author’s work 
entitled Practical Electrical Testing for first and second year 
students. It may, however, here be remarked that when using 
the potentiometer for measuring current, resistance, and high 
voltages, the principle, as is well known, consists in reducing 
any of the three electrical quantities which are to be measured, 
to the form of electrical pressure, or E.M.F., so that it can be 
compared by means of a potentiometer with a standard pressure 
such as that of the Clark cell. 


ELEOTEIGAL ENGINEERING TESTING 


507 


The N.C.S. Potentiometer. 


This is a somewhat new type of potentiometer, in which there 
is no slide wire to get injured or deteriorate with time. It works 
entirely by means of adjusted resistances of perfectly definite 
values, the ends of which are permanently attached to circular 
rows of contact studs. 

Fig. 205 shows a general view of the potentiometer in its 
containing case with the lid slightly raised. The internal 





Fig. 205. 


arrangements and connections are seen in the symbolical diagram, 
Fig. 206, with reference to which the working of the instrument 
will be understood more clearly. The two diagrams exactly 
correspond with one another so far as the relative positions of 
the various parts are concerned. 

A secondary battery is joined to the two terminals F, and 
sends a current through the two dials G and D in series and 
then through adjusting resistances K and H. The G dial has 
150 exactly equal coils in it, numbered from 0 to 150. The 
D dial has 100 equal coils in it, the whole dial being exactly 
equal to the one coil in G. The two together are therefore 




































































508 


ELECTRICAL ENGINEERING TESTING 


Gfjuivalent to a slide wire with 15,000 fixed contacts at equal 
distances. The dial K contains nineteen equal small wire 
resistances, and II is a carbon resistance for fine adjustment 
of the current, so that working with one secondary cell, the 
potentiometer can be set to read volts direct with a Clark cell. 

A standard cell of known E.M.F. is joined up to A + and A —, 
and the switch L is put over to A. 

Any convenient galvanometer, the most convenient form being 
a D’Arsonval, is joined up to the galvanometer terminals. 

Say the voltage of the standard cell, at the temperature used, 
is 1*4412; the arm of the G dial is then set to 144, and that of 
the D dial to 12. 

The galvanometer key is then depressed on to its first stop 
(taking care that the catch is underneath so that it cannot go 
right down) and the outside resistance K is adjusted till—with 
the switch on one stop—the deflection is one way, and, with it 
on the next, in the opposite direction. The head II is then 
turned until an exact balance is obtained. A slight pressure 
should be put on the bead II when turning it; it may not be 
found necessary to use II at all the next step. 

Push aside the catch on the galvanometer key and depress fully. 
This cuts out a J megohm which was previously in circuit to 
protect the cell. 

An exact balance can now be obtained by further adjustment 
of II or possibly K. 

The instrument is now set so that each division on C is equal 
to *01 of a volt, and each division on D is equal to *0001 volt. 

Any low E.M.F. to be measured is joined to BB terminals and 
the switch L set to B ; for example, a Leclanch^ cell or the 
terminals of a low resistance for current measurement. 

The galvanometer key is then again fully depressed and a 
balance obtained by moving G and D. 

The E.M.F. between B and B is then read direct on the two 
dials; for example, if the reading on G is 83 and that on D is 
67, the volts between B and B is *8367. Any volts higher than 
1*6 have to be measured on the terminals marked volts + and 
—, and with the switch L turned to V. The switch M is then 
turned to any convenient multiplying power, 3, 10, 30, etc., 
and the readings obtained from G and D (when the balance is 


ELECTRICAL ENGINEERING TESTING 


509 


obtained as before) have to be multiplied by this multiplying 
power; for example, if G is 103 and I) 26, and M is standing 
at 30, the volts on the terminals are 1’0326 x 30 or say 30‘98. 

The resistances in H and K are arranged so that any voltage 
up to 3 volts can be used at F, thus allowing either a secondary 
cell or two large Leclanches to be used; it is well to leave the 
battery joined up to the instrument for some ten minutes or so, 
60 as to steady down before beginning work. 

A set of coils are required to potentiometer down high voltages 
amounting to 300 so as to bring them within range of the dials. 



The three-way switch L according to its position inserts the 
known fraction of the high voltage, the standard cell, or any 
third unknown E.M.F. in circuit with the instrument. 

The following points should be noted— 

When using a standard cell, get a balance with the catch 
under the key before removing the catch. 

Be careful to join up any cells, etc., to their right terminals 

and not + to —. 

Do not screw the head H down too tight. It draws up a rod 
which compresses a carbon resistance*; there is plenty of range 









































510 


ELECTEIGAL ENGINEERING TESTING 


to cover the difference between two stops K without squeezing 
it at all tight. 

Press head H down when turning; it keeps the resistance 
steadier. 

The total resistance in the M dial is 100,000 ohms; not more 
than 200 volts should be applied to volts terminals for any length 
of time. For higher values a known resistance can be added 
outside and allowed for. 


Crompton’s Potentiometer. 

This potentiometer is shown diagrammatically in Fig, 207. It 
consists of a wire AB stretched over a scale and through which 
a constant current is maintained from one or more secondary 
cells of sufficient size to keep it fairly constant for three or four 



days’ working. A variable resistance G and rheostat G^ is intro¬ 
duced in series with the cell and wire, arranged so as to adjust 
this current to give a certain difference of potential at the two 
ends of the wire. The circuit which includes the E.M.F. to be 
measured is connected to one end D of the effective potentio¬ 
meter wire, and through a galvanometer to a sliding saddle C, 
which carries a knife edge to make contact on the wire AB 
at any desired point of its length. This circuit is so coupled 

















ELECTRICAL ENGINEERING TESTING 


511 


up that the E.M.F. to be measured is opposed to the E.M.F. in 
the portion of the slide wire EG, in order that the position of 
C on the slide wire can be adjusted until the two E.M.F.s 
balance one another so that no current passes through the 
galvanometer. In order to compare the E.M.F. to be measured 
with the standard E.M.F., a Clark cell is put in in circuit with 
the galvanometer, and the position of the slide G is noted when 
the above-mentioned balance has been obtained. The electrical 
pressure or E.M.F. required to be compared is then substituted 
for the Clark cell, and a similar balance found by adjusting the 
slider G a second time. Then the comparison between the two 
E.M.F.s can be made by comparing the respective lengths of 
the slide wire included between the points B and G in the first 
and second case. By dividing the slide wire AB into a suitable 
number of parts, and adjusting the variable resistance until the 
galvanometer comes to zero when the Clark cell is in circuit and 
the contact C is at a point on the scale corresponding to the 
temperature value of the Clark cell, say 1’434 at 15° C., the 
instrument becomes direct reading in volts or fractions of volts. 

Dr. Fleming in the year 1883 first called attention to the 
advantages of this system of measuring, and since that time the 
system has been steadily developed, and refinements have been 
introduced. As the following description will apply to instru¬ 
ments such as are suitable for a municipal standardizing 
laboratory, it is here necessary to specify what are the require¬ 
ments and limits of accuracy within which these instruments 
may be reasonably expected to measure. First comes the verifi¬ 
cation of voltmeters. This is of importance on account of the 
disputes that are likely to arise as to the pressure supplied from 
electric-lighting stations. Voltmeters for standard pressures of 
150 to 200 and 220 volts are principally used for noting the 
pressures in the consumers’ houses, and for such cases it is 
desirable that their readings at about the standard pressure 
should be verified to one-tenth of a volt or within one part in 
1000. Next it should be possible to verify and. certify the 
constant of the various kinds of meters by which the electrical 
supply is measured to the consumers. They also ought to be 
verified to one part in 1000. Next comes the verification 
of ordinary ammeters, or current instruments used for trade 


612 


ELECT RIGA L ENGINEERING TESTING 



purposes to about the same degree 
of accuracy. In both these last cases 
the range through which instru¬ 
ments would Lave to be compared 
is very considorable; for instance, 
the instruments sent for verification 
may be those used for testing electric 
lamps or for telegraphic purposes, 
measuring currents of one milli- 
ampere, up to those used for metal¬ 
lurgical or electrolytic purposes up 
to 5000 amperes or more. 

The correct comparison of resist¬ 
ance standards as well as the cor¬ 
rectness of the ratios of resistance 
boxes ought to be capable of verifi¬ 
cation to one part in 10,000. In 
the comparison of resistances must 
be included the testing of the con¬ 
ductivity of various metals, and the 
insulation resistance of various insu¬ 
lating materials. 

In the first form of instrument 
proposed by Dr. Fleming, the poten- . 
tiometer wire was made of an alloy 
of platino-iridium four metres long, 
and had a resistance of about 23 
ohms. The wire was divided into 
two parts and stretched over a scale; 
the whole of the wire, being re¬ 
quired for measuring purposes, had 
to be carefully calibrated, that is 
to say, its electrical resistance made 
equal per unit length throughout 
its entire length. This was a very 
tedious and expensive process. It 
was found very difficult, if not im¬ 
possible, to obtain wire as it finally 
left the drawplate, which was suffi- 











































ELECTRICAL ENOINEERING TESTING 


513 


ciently homogeneous to be used without further adjustment. 
In most cases the whole of the wire had to be carefully scraped 
or rubbed until the required equality of resistance was obtained. 
Such a wire was very valuable, and if it was broken or acci¬ 
dentally melted, its loss was a serious matter. 

In a later form of instrument (Fig. 208) Messrs. Crompton 
have abandoned the use of this expensive material, and at the 
same time have arranged so that only one-fifteenth part of the 
wire AB is stretched over the scale and subject to the wear of 
working. AB in the drawing shows this portion of the wire 
25 in. long, stretched over a scale divided into 1000 parts; 
therefore each division on the scale being one-fortieth of an 
inch, represents one fifteen-thousandth of the pressure at the 
two ends of the wire ; in other words, if the instrument being 
standardized to 1*5 volts at the terminals of its wire, each of 

1*5 

the smallest divisions of the scale represents qqq - or one ten- 


thousandth part of the volt. The resistance of this wire is 
usually about 2 ohms, and the remaining wire is divided into four¬ 
teen coils, each of about 2 ohms, so that the resistance of the 
whole is 30 ohms. These coils are naked spirals, the terminals of 
which are fixed to the underside of the fourteen contact blocks 
shown at E. The swinging arm shown can make contact with 
any one of these blocks. It is an easy matter to adjust these four¬ 
teen coils when the instrument is first made so that their resist¬ 
ance will accurately equal one another, and the resistance of the 
working wire AB can then be adjusted by slightly stretching it 
until it is also exactly equal to any of them. As the fourteen coils 
are protected they never need further adjustment, but if in the 
course of time the exposed portion of the wire AB becomes worn 
or is in any way damaged, a new piece of wire can be substituted 
in a few minutes and stretched by means of the stretching screw 
shown at F, until its resistance over the portion from 0 to 103 
exactly balances that of any one of the fourteen coils. G is the 
sliding saddle carrying the contact. This consists of an ebonite 
box arranged to slide smoothly on the scale. It is provided with 
a knife-edge spring contact so that the contact may be made 
with a regular pressure which is independent of the pressure of 
the hand of the user; it also has a micrometer adjustment for 


L L 



514 


ELEGTRIGAL ENGINEERING TESTING 


accurate work. The semi-circular switch G and the cylindrical 
rheostat G^ shown to the left, which latter gives a graduation 
of resistance, form part of the variable resistances above described, 
and which are required to reduce the difference of potential 
between the terminals of the wire from that of the secondary 
cell 2 volts to any desired value. The value that has been 
chosen in this instrument is 1’5 volts. The instrument is 
provided with four pairs of terminals, 1, 2, 3, 4. A Clark cell 
can be put on to one of these, and any three other electrical 
pressures to be measured can be connected on to the others. 
The switching arrangement II shown in the centre brings the 
galvanometer into series circuit with any one of these pairs of 
terminals. The contact key K shown to the extreme right is 
used for short-circuiting the galvanometer. 

Whenever it is required to measure an electrical pressure 
greater than that on the terminals of the potentiometer, that is 
to say, in this case, greater than 1*5 volts, the pressure to be 
measured is applied to certain terminals of a resistance box, 
and the terminals of the potentiometer are connected to other 
terminals on this box, which include between them a resistance 
which is an even part, say, one-tenth, one hundredth, or one 
thousandth of the entire resistance of the box. 

When, however, the potentiometer is employed for measuring 
currents, this is done by measuring the difference of pressure at 
two points on standard resistances which, in order to make the 
instrument direct-reading, must be either 1 ohm, one-tenth, one- 
hundredth, or one-thousandth and so on. As these low resistances 
have often to carry very high currents, they have to be designed 
so that they do not heat to a sufficient extent to introduce errors. 
A description of a few different forms of them will be found 
on p. 604. 


Method of “ Setting Potentiometer ” by Standard Cell. 

One secondary cell being connected direct to the extreme left- 
hand pair of terminals, the galvanometer to those on the extreme 
right and a standard Clark cell to terminals IV suppose. Note 
the temperature of the Clark cell on its own thermometer, and 
from the table showing its E.M.F. for different temperatures 


ELECTRICAL ENGINEERING TESTING 


515 


affixed to the instrument or by calculation, using the formula on 
P. 17, or table p. 643, obtain its present E.M.F. corresponding 
to its present temperature. For example,. suppose this to be 
15° C., then the E.M.F. = 1*4340 volts. 

Next place the double switch H on studs 4, the lever on stud 
14 of and the slider key C at 340 on the scale, then pressing 
this latter, adjust the resistance G and rheostat 6^^ so that the 
galvanometer comes back to the zero, at which it was originally 
set. Now every one of the scale divisions will be equal to one ten- 
thousandth of a volt, and the potentiometer is thus “ setJ^ To use 
the instrument for voltage measurements H is turned to the un¬ 
known E.M.F., while G and (rj remain untouched, only and C 
now being varied to obtain balance with the unknown E.M.F. in 
circuit with the galvanometer. 

Precautions. —A high resistance, such as 10,000 ohms, should 
always be connected up in series with the standard cell before 
placing this across the terminals of the potentiometer, but when 
balance in the “ setting ” is practically obtained, the resistance 
can be cut out of circuit to make the balance as sensitive as 
possible. By doing this the cell will not be able to send any but 
an extremely small current when the balance is far from perfect, 
and only under such conditions of use can the cell be relied on as 
an accurate and constant standard of E.M.F. of the value set forth 
in the table above mentioned. 

In taking a series of readings with a potentiometer, care 
should be taken, at intervals, to see that the “ setting ” with the 
Clark cell remains constant, and if it doesn’t, to adjust so that 
it is. 

Sources of Error. —The current from the secondary or “ Working 
ceir* through the stretched wire and resistances A owing 

to the E.M.F. of this cell varying. To prevent this it is advisable 
not to use a newly-charged cell, the E.M.F. of such being liable 
to frequent alteration, but to use one amply large enough, that 
has already been J discharged. An extremely constant E.M.F. 
and current will then be obtained. 

Again, a further error will be introduced by the stretched wire 
not being uniform, and great pains should be taken to ensure that 
it is uniform. This may be proved by calibrating it carefully in 
the manner employed in thus testing a metre bridge wire, a 


516 


ELEGTRIGAL ENGINEERING TESTING 


full description of which is given in the author’s work on Practical 
Electrical Testing ; or by sending a perfectly steady and constant 
current through the wire and measuring the fall of potential down 
equal lengths throughout the scale. 

Still another error may be caused by leakage causing the 
galvanometer to deflect when an accurate balance has been 
obtained. To avoid this, the insulating of the various pieces of 
apparatus should be attended to. 

The above form of potentiometer has been much improved by 
the makers, and that made at the present time differs from the 
above in several constructional details. These, together with 
the appearance and connections of the latest type of instrument, 
will be easily understood from the following description by the 
makers. 

The construction of the potentiometer itself is shown 
diagrammatically in Eig. 209. The calibrated wire is arranged in 
fourteen coils, called potentiometer coils, lettered AB, and a 
straight section BG^ called the scale wire, the resistances of the 
several coils and of the straight section being equal. One sliding 
contact Q moves over the terminals of the fourteen coils, and 
another R along the straight wire. The reading of the instru¬ 
ment in the position shown is 1‘046. The pairs of points whose 
potential differences are to be compared are connected to the 
blocks of the double-pole switch if, whose levers, MN, connect 
them, one pair at a time, to the sliding contacts QR through the 
galvanometer. The galvanometer key II is arranged to complete 
the circuit through two resistances, which are short-circuited in 
succession as the key is depressed. The current required is de¬ 
rived from a small secondary battery G. An adjustable resist¬ 
ance, consisting of a set of coils DE^ and a continuous rheostat 
F is placed in the circuit. By adjusting these the resistance of 
the circuit and the current passing through it from the storage 
cell, and consequently the fall of potential along the scale wire 
can be continuously altered, and the operator is able to obtain a 
galvanometer balance against a standard cell when the reading of 
the sliders is that of the known E.M^F. of the cell at its actual 
temperature. If, for example, the temperature of the cell be 
15°, so that its E.M.F. is 1*434 volts, the sliders may be set 
to that reading, and the galvanometer brought to zero by 


a 


H 


rO 6 


AAAAAAA/ 

I 

VWWW\ ■ 


o 


O 

O 

* 

CO 

o 














































518 


ELECTRICAL ENGINEERING TESTING 


adjusting the resistance EE and the rheostat E. When this 
has been done the scale readings at all points are direct readings 
in volts. 

A view of the potentiometer is given in Fig. 210, and a diagram 
of its internal connections in Fig. 211. Here a 6 is the scale wire; 
c the set of equal potentiometer coils in series with it; c? is the 
double-pole switch connecting the six pairs of terminals ABGDEF 
in succession to the slide contacts; e f are the resistance coils 
and rheostat respectively, and G is the galvanometer key. All 
the moving contacts are under glass, and the coils and the scale 
wire are inside the box. The box itself is completely closed, but 
the inside can be inspected by removing a sliding bottom. 
Nearly all the measurements made, involve the use of a standard 
cell, and one pair of terminals, the pair A, is assigned to its 
connections to save confusion in working. Fuses of fine wire 
are inserted at all terminals except those for the galvanometer to 
save the instrument coils in the case of an accidental connection 
to a source of high pressure. 

Two scales are engraved for slide wire readings. One is a series 
of even divisions from 0 to 105, the resistance of the scale wire 
between 0 and 100 being the same as that of each potentiometer 
coil. It has been found convenient to be able to take readings 
a little beyond the 100 mark without having to move the potentio¬ 
meter coil switch, and the scale is extended to 105 to admit of 
this. The other scale gives the values of the Clark cell at different 
temperatures, Tmd is used in the following way :—The potentio¬ 
meter coil switch is set to 14, and the slide to the temperature of 
the Clark cell taken from the thermometer attached to it. The 
potentiometer reading is then the correct value in volts of the 
Clark cell at that temperature. By adjustment of the rheostat 
the galvanometer is balanced, and when this has been done the 
current in the potentiometer wire is such that readings at all points 
give correct values in volts, and the instrument is a direct-reading 
voltmeter. Its maximum range is then 1-5 volts, reading in 
thousandths of a volt, and by inspection to ten thousandths. 




































































520 


ELEGTEIGAL ENGINEERING TESTING 




) 































ELEGTRIGAL ENGINEERING TESTING 


521 


Potentiometer Volt Box. 

This is a simple and most convenient piece of apparatus by 
means of which a known definite fraction of any voltage can be 
easily and quickly obtained. Hence such an instrument can 
be employed with a potentiometer for the measurement of high 
voltages, enabling the small requisite voltage to be taken off 
and used for measurement in the potentiometer. Fig. 212 shows 
a general view and Fig. 213 (p. 523) a plan of the internal connec¬ 
tions of a variable volt box designed by the author for use with a 
Crompton potentiometer. It consists of two sets of resistance 
coils, each set having its own separate semi-circular row of con¬ 
tact studs over which the spring contact levers work and make con¬ 
tact. Two pairs of terminals are provided, the high E.M.F. being 
directly connected to those marked M, which are larger terminals 
than those marked B to distinguish them and avoid mistakes. 
B is the side at which the known fraction of the total P.D. is 
obtained, and of course go to the potentiometer direct. With 

the levers as shown. Fig. 213, g 9 oV +10 0 or of the total P.D. 

across M will be taken from B, 


Low Resistance Measurer. 

The following instrument affords a simple and convenient 
means of measuring very low resistances, such as are met with in 
the armatures of large dynamos, motors, and electric light mains. 
These cannot be obtained by an ordinary Wheatstone Bridge, 
owing to the difl&culty of making, and the resistance introduced 
by, the contacts, and other reasons which go to vitiate and make 
the results worthless (see Fig. 214). 

The working of the instrument is as follows—The resistance to 
be measured is joined in series with a battery and the slide wire. 
One coil of a differential galvanometer is joined by two leads to 
the two ends of the resistance to be measured, and the other coil 
across more or less of the slide wire. 




522 


ELECT RIGAL ENGINEERING TESTING 


The wire is divided into 1000 parts, and the whole is so 
arranged that the fall of potential over the whole wire, through 
the one galvanometer circuit, exactly balances that over y^^h 
ohm in the other. 

The readings are then proportional throughout the scale. By 



Fig. 212. 


shifting two plugs the values may be multiplied by 5 or divided 
by 2, thus making the top read ^ or of an ohm. 

Directions for use. —Place the instrument on a fairly level table 
or bench. 

Free the galvanometer needle by turning the screw at the back. 

Turn the galvanometer on its shank till the needle points tu 
zero. 















ELEGTRIGAL ENGINEERING TESTING 


623 


Join the large terminals of the instrument in series with the 
resistance to be measured, and a cell capable of giving, say, 5 
amperes; introducing somewhere in the circuit a resistance, to 
bring the current down to about 5 amperes. 

Any odd bit of iron or German silver or other wire will do. A 
piece of G.S. suitable for use with a two-volt cell is sent with the 
instrument. 

Join the long leads on to the small pair of terminals. 

Depress the key on the contact arm to touch the wire; close 
the circuit switch, and the direction of the deflection. 



Pi •ess the two contact spears at the ends of the long leads on to 
the two points between which the resistance is to be measured— 
the contact arm being up. Again close the circuit switch and 
note direction of deflection. If in same direction as before, 
reverse the two contact spears. 

Then, keeping the spears pressed on the resistance to be 
measured, depress the key on the contact arm. 

If the deflection reverses it shows that the fall of potential 
over R (the unknown) is less than that over the wire, and the 
arm must be moved back towards the zero end until the galvan¬ 
ometer needle points to zero. 

The arrow on the contact arm then points to the resistance on 









524 


ELEOTBIGAL ENGINEERING TESTING 


the scale direct in ten-thousandths of an ohm, thus a reading of 
is or-0517. 

When the plugs are in 2 and 3 the instrument reads as 
above described. When in 1 and 2 the reads must be multi¬ 
plied by 6, thus a read of 274 = *0274 x 5 =’1370 ohm. When 
in 1 and 3 the reads must be divided by 2, thus 98 = *0098 2 

= '0049 ohm. 

The spears should make fair metallic contact, but nothing more 
is necessary. 

The galvanometer turns on its pillar, and can thus be set to 
zero. It should be set to zero with the current switch closed. 



Fig. -21 i . 


but without the contact arm being pressed or the spears in 
contact. This avoids all disturbances from the heavy current 
leads. 

The contacts in the current leads do not need any care, their 
resistance, even if variable, does not affect the result. 


Approximate Tests for very Low 
Resistances. 

In armatures it is sometimes desirable to test each bar separ¬ 
ately, to see that they are all the same, but if tested as above the 
reading would be so very small that there would be no certainty. 
A comparison of the bars can be taken by putting a shunt of any 










ELEGTRIOAL ENGINEERING TESTING 


525 


convenient size, say 6" of No. 8 platinoid, across the heavy term¬ 
inals T T Sit the end of the slide iron; using a larger current and 
working as before. The results will be of course only compara¬ 
tive, but if the shunt is firmly fixed, the readings will be quite 
sufificiently accurate for practical purposes. In the same way 
any two very low resistances can be compared by putting the 
two in series with the instrument and then transferring the point 
contacts from one to the other. 


Siemens Low Resistance Bridge. 

Instructions for use. —Connect up as shown in Fig. 215, which 
is a symbolical representation of the bridge and its connections, 
the general view being shown in Fig. 216. 

In each of the arms of the branch box marked x , unplug equal 
resistances and also on the — side, the resistances being chosen 
according to the magnitude of the resistance to be measured. 

The slide wire resistance is carefully calibrated and is com¬ 
pensated for all changes of the air temperature so that 0 to 100 
= O’Ol standard ohm. (At any time should the wire require 
cleaning, only chamois leather should be used for the purpose.) 

The contact slide vices, etc., are arranged for testing conduct¬ 
ivity, but any other resistance may be tested on jointing it by 
four connection leads to LI, Ly and X2, Lz ] L\ and Z2 being 
the current connections ” and Ly and Lz the “ potential connec¬ 
tions,” it must be remembered that any excessive resistance in 
the leads from the potential contacts to the branch box would 
increase the value of the branches and should be allowed for; 
they should therefore be as low as conveniently possible. The 
battery used should be one of low internal resistance ; the galvan¬ 
ometer serves to indicate that there is a current flowing in the 
circuit. 

Resistance Test. —The left-hand vice, etc., being clamped at 
zero, insert the specimen and clamp it at both ends; the length 
corresponding to the resistance measured will be given by the 
pointer on the meter scale, and is the distance between the two 
knife edges or “ potential contacts.” Alter the position of the 
sliding roller contact on the resistance wire until after closing 


526 


ELEGTRIOAL ENGINEERING TESTING 


the battery circuit by means of the key B, and then pressing 
key Mf no motion of the galvanometer needle is observed; the 



O 


position of the roller contact being then read off, will give the 
resistance on the slide wire (S). 

Then the resistance of the specimen is X= aS' . — ohms. 

















































































ELECTRICAL ENGINEERING TESTING 


527 


i' 


Example. —If the two tens be unplugged 
on the X side and the two thousands on 
the side and S — 45*5, then 

X lih X 0-01 = 0-0000455 
standard ohm. 


CO 

(M 

6 

M 


Conductivity Test of Copper Wire. —For 
determining the conductivity of copper 
wire, the wire where it will come in con¬ 
tact with the vices and knife edges must 
be cleaned from all oxide. Then insert 
the end in the left-hand vice and clamp 
it. The right-hand vice must now be 
set by the pointer opposite the tempera¬ 
ture of the specimen, as shown by the 
small scale, which is graduated from 45° to 
75° F. The distance between the knife 
edges when the pointer is opposite 60° F. 
is 816-06 m.m., and the scale is calculated 
so that the difference in length com¬ 
pensates for the difference of resistance 
due to temperature of the copper. 

Proceed as before to measure the resist¬ 
ance (X), taking note of the exact length 
in millimetres as shown by the metre scale. 
Then cut off by means of the knives and 
ascertain its weight in grammes, and by 
simple proportion determine the weight 
of 816*06 m.m. (IF). The resistance of a 
pure copper wire 816*06 m.m. long weigh¬ 
ing 1 gramme is 01 standard ohm @ 60° F. 
4'hen the conductivity per cent, of pure copper is 

0-1 




N X W 


X 100 . 


Example— 


Lengtli 

Weight in 

Weight of 

Resistance 

Value of 

% 

cut off 

grammes 

816 00 m.m. 

X 

X X. W 

conductivity 

toy in. 111 . 

14-63. 

14-76. 

0-00696. 

0-1027. 

97-33. 























































































528 


ELECTRICAL ENGINEERING TESTING 


Amsler’s Planimeter. 

If any figure on paper is measured in the ordinary way with 
compass and rule, the figure is first divided into triangles the 
area of which can be calculated, and the sum of their areas will 
give the area of the figure. 

This method was shortened very much in 1827 by Mr. Oppen- 
koffer, a Swiss engineer, who invented an instrument called the 
“ Planimeter,” which measured the area of plain surfaces by 
following the outlines of the figure with a pointed tracer, which, 
being connected with a dial-plate, showed the area of the figure. 
This instrument soon came into general use, although somewhat 
awkward and expensive. 



Fia. 217. 

In 1849 an improvement was made by Mr. 'W’elty, another 
Swiss engineer, which was still rather clumsy. No further 
improvement was made till 1854, when Mr. J. Amsler, Professor 
of Mathematics at Schaifhausen, introduced the Planimeter 
which is now in use; the construction is simple, the instrument 
can be carried about without fear of damage. One experiment 
showed that an area which after dividing it into triangles took 
nearly three hours, was done by Amsler’s Planimeter in about 
two or three minutes. 

Instructions for working the Planimeter. 

(1) Before working with the instrument, adjust the screw 
centres upon which the index roller D revolves, so that the roller 
works freelv, and does not touch the vernier. The same care 
must also be taken with the centre pin G. It is good to grease 













I^LEGTIUGAL ENGINEERING TESTING 


529 


the screw centres now and then, so that they work easily. Care 
should be taken to prevent the tube B, the tracer E, and the 
point E from being bent, and also to see that the barrel D is kept 
uninjured. 

(2) To find the area of any figure, set the roller D and the 
counting wheel G to zero ; the square rod A must be pushed 
into the tube B, and the line on A marked 1 sq. dem., or OT sq. 
ft. etc., must come even with the small line on the bevelled part 
of the tube B ; when this is done, place the instrument on the 
paper, and see that the roller Z), the tracing point F, and the 
needle point E touch the paper. Press the point E slightly into 
the paper, and put the small German silver weight on the hole 
over the point E ; the instrument is then ready for work. 

(3) Take any point P on the outline of the figure about to be 
measured, set the tracing point F to that point, and when it is 
marked, read off the index roller D and counting wheel G. For 
example, suppose the counting wheel G shows 2, the roller Z) 91, 
and the vernier 5, the number will be 291’5. Follow the outline 
of the figure with the point F as accurately as possible to the 
right, until you come to the starting-point. Straight lines can 
be followed along a ruler; then read off the numbers on wheel 
and roller; say it is the second time 476‘7. 

(4) When these two numbers are obtained, there are two cases 
to be observed— 

(1st) If the point E is outside the figure, subtract the first 
reading 291*5 from the second 476*7, the remainder is 185*2, 
which shows that the area contains 185*2 units. Of course the 
units depend entirely on the regulation of the bar A ; if they are 
0*1 sq. ft. we have 185*2 x 0*1 = 18*52 sq. feet, as the area of 
the figure measured on the paper. 

The rule therefore is, when the point E is outside, multiply the 
difference of the two readings by the number on the bar to the 
right of the corresponding division. 

(2nd) When the point E is inside the figure, before making 
the subtraction, the number engraved on the top of bar above 
the corresponding line of division, must be added to the second 
reading. In this instance, suppose the number on top of bar A 
is 20*985, the second reading is 4*767, the calculation would be 
as under— 


M M 


530 


ELECTRICAL ENGINEERING TESTING 


2nd reading = 4‘767 

Number over O'l sq. ft. = 20 985 

25-752 

Deduct 1st reading = 2-915 

Remainder 22-837 


The area is therefore 22-837 units or 22*837 x 0-1 = 2-2837 
square feet. It is of no consequence whether the roller is inside 
or outside the figure, provided it is on the same level. 

(5) In measuring large figures, it may sometimes happen that 
the wheel G goes through one or two or more entire revolutions. 
If such is the case, 10,000 or 20,000, etc., must be added to the 
difference of the two readings before multiplication. 

There is another form of planimeter which measures surfaces 
in square inches only; it is more simple than the other in con¬ 
struction, and can bo worked with the above directions, always 
bearing in mind that the result is shown on the counting wheels 
in square inches and not as in the other instrument in O’l square 
decimetres, or O'l square feet, etc. 


Amsier’s Planimeter for Determining the 
Mean Pressure in an Indicator Diagram. 

By the use of this instrument a great saying of time is effected 
in calculating large numbers of indicator diagrams, and the 
results obtained are more accurate than by any other method. 

Directions for using the Planimeter. 

The diagram is carefully pinned to a perfectly flat board. The 
points 00 of the planimeter are adjusted so that the distance 
between them is equal to the length of the diagram projected on 
to the atmospheric line. The instrument is then placed upon the 
board, the point h being brought into agreement with any fixed 
point of the diagram, while the weighted point c is placed in any 
convenient position outside the diagram. The point h is then 
passed once round along the lines of the diagram. The indication 






ELEGTRIGAL ENGINEERING TESTING 


531 


on the wheel P and disc S, when the point b has again reiiched 
the starting-point, divided by 40 gives the mean height of the 
diagram in inches. Supposing, for instance, that the wheel P 
was adjusted to stand at zero before using the instrument, and 
that, after tracing the diagram, the disc S —which advances by 
one figure for every ten revolutions of the wheel P —stands 
between 1 and 2, while the wheel P indicates 21*2, a vernier being 
provided for reading the last figure, then the resulting number is 
121*2, and this divided by 40 gives 3*03, which represents the 
mean height of the diagram in inches. Now, supposing that the 
diagram was taken by means of a No. 7 Richards Indicator 
Spring with a scale of 32 lbs. to the inch, then the mean pressure 
amounts to 3*03 x 32 = 96*96 lbs. per square inch. 

In practice the calculation is somewhat simplified, as the 
springs used are mostly of such scales that instead of dividing by 
40 and multiplying by the vertical scale, the mean pressure may 
be obtained by simply multiplying by a factor corresponding to 
the scales used. 

In order to secure accurate results, the instrument must be 
carefully cleaned before being used, and the board must be 
perfectly flat. 


Thompson’s Indicator. 

The chief distinguishing feature of this Indicator consists in a 
novel parallel motion which is preferable to the motion employed 
in the Richards Indicator on account of its greater lightness and 
rigidity. The irregularities in the diagram due to the inertia of 
the moving parts are consequently greatly reduced, and a figure 
is obtained which forms the nearest possible approximation to the 
correct diagram. The parallel motion is carefully designed to 
ensure that the pencil point describes a straight line, and that the 
motion of the pencil point is precisely proportional to the dis¬ 
placement of the indicator piston throughout the stroke. 

Owing to its general efficiency, this indicator is also particularly 
applicable for high speeds, and it has been successfully employed 
at a speed of 400 revolutions per minute. 

The piston is rigidly connected with the piston rod, which is 
guided in the cylinder cover, and in this way a perfect guide is 


532 


ELECTRICAL ENGINEERING TESTING 


obtained for the piston. The tension of the spring in the drum 



can be varied to suit the speed of the engine by loosening the 


Fig. 218. 

























































ELEGTIUGAL ENGINEEllING TESTING 


538 


nut on the top of the drum spindle and by turning the disc hold¬ 



ing the spring until the required tension is obtained, tlie nut 
being then again screwed home. 































































































































































































































































































































































534 


ELECTRICAL ENGINEERING TESTING 


Table YIIT. 

List of Springs for Thompson’s Indicator. 


Sizes, No. 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

For Pressures 

-15 

-15 

-15 

-15 

-15 

-15 

-15 

-15 

-15 

-15 

-15 

-15 

-15 

8 

12 

18 

30 

40 

50 

68 

75 

95 

120 

150 

180 

200 

Scale: 1 lb. per sq. inch 
equals .... inch 

1 

8 

tV 

iN 

tV 

1 

Tcr 

■sV 

1 

■57 

1 

•S7 

1 

■77 

1 

17 

1 

77 

1 

T7 



Small Thompson Indicator. 

This indicator is specially adapted for indicating high-speed 
steam engines, gas engines, etc., and will give correct diagrams 
at all engine speeds occurring in practice without the necessity of 
taking special precautions, and has the further advantage of 
being very portable. It is, therefore, eminently suited to the 
requirements of the engineer or engineering student. The 
apparent disadvantage of a somewhat smaller diagram obtained 
from this indicator at slow speeds is more than compensated for 
by increased accuracy. 


Table IX. 

List of Springs for Thompson’s Small Indicator. 




Sizes, No. 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

For Pressures from 

fLbs. per\ 

-15 

-15 

-15 

-15 

-15 

-15 

-15 

-15 

-15 

-15 

-15 

0 

0 

0 

0 


up to1 

sq.inch. f 

6 

15 

20 

30 

45 

60 

70 

80 

100 

125 

150 

200 

250 

300 

375 

Scale: 1 
equals 

lb. per 

sq. inch 

1 

T7 

1 

T7 

1 

57 

1 

■77 

1 

77 

1 

■77 

1 

77 

1 _ 

6 

1 

77 

1 

77 

1 

77 

1 

T77 

T77 

1 

577 

1 

777 


The Silvertown Portable Testing Set. 

This is a small collection of the necessary instruments for 
testing electrically such insulated conductors as are used in tele¬ 
graph, telephone, or electric-light work. 

The whole set is contained in two small wooden boxes, of which 
one holds the batteries and the other the galvanometer, resistance 
coils, key, and commutators required for making the two most 
important measurements on such circuits. These are measure- 
































































ELECTRICAL ENGINEERING TESTING 535 


ments of the resistance of the conductor, and the efficiency of the 
insulation, 

T he battery consists of two parts ; one—commonly called the 



bridge battery—is a set of three Leclanch^ cells of low resistance 
intended to be used in testing conductor resistances only, a pur¬ 
pose for which currents of electricity of sensible magnitude are 


























































































5T) ELECTRICAL ENGINEERING TESTING 

required. The other part is a set of 36 small Leclanche cells 
having a total electro-motive force of 55 volts, intended exclusively 
for measuring insulation resistances, or other resistances, of con¬ 
siderable magnitude. These cells are designed to give only very 
small currents of electricity, and care should be taken not to 
connect them inadvertently to the Wheatstone Bridge or other¬ 
wise put them on a circuit of low resistance. This battery, called 



Fig. 221 . 


the insulation battery, is subdivided into three sections of 3, 15, 
and 39 cells, so that electro-motive forces of about 5, 25, or 60 
volts can be employed as may be found convenient. 

For connecting the battery to the testing instruments con¬ 
venient leads are provided terminating in brass plugs with in¬ 
sulated handles for inserting in the proper plug-holes. The 
instruments shown in perspective in Fig. 221 are connected up 
together in their own box in such a way as to secure the greatest 

































































































ELECTRICAL ENGINEERING TESTING 


537 


portability and economy of space, and to enable the two tests to 
be taken with the greatest readiness. 

A plan of this box showing the general arrangement of all 
the connections, resistance coils, and galvanometer is seen in 
Fig. 222. 

The galvanometer consists of a coil of fine wire on a brass 
bobbin, in the centre of which a small magnetic needle with an 
aluminium pointer is hung in the same way as is usual in com¬ 
passes. The pointer projects through the opening in the end of 
the coil, and the excursions of the needle are limited by the size 
of the opening to about 45° on each side of the centre. On re¬ 
moving the glass cover the needle on its point may be taken out 
by withdrawing the slide on which it is pivoted from inside the 
coil. The scale, which is a scale of equal currents, is approxim¬ 
ately a scale of tangents, and is obtained empirically by calibrating 
the instrument. The north end of the magnetic needle points 
to the left-hand side of the box when it is swinging freely in its 
zero position. 

On the left-hand side of the box is placed the controlling 
magnet, and the position of this affects the sensitiveness of the 
galvanometer. When the north pole of the controlling magnet 
is uppermost, the galvanometer will be most sensitive; on turning 
the magnet round, so that the south pole is uppermost, the de¬ 
flection of the needle due to any given cuirent will be reduced 
by about 40 per cent. Generally in testing the insulation of 
well-insulated wires, the galvanometer is required to be as sensi¬ 
tive as possible, and the north pole of the controlling magnet 
should be at the top; but for measuring conductor resistances, 
for which the galvanometer is generally amply sensitive, it will 
be found more convenient to bring the south pole uppermost, 
thereby causing the galvanometer needle to oscillate more rapidly. 

Besides thus affecting the sensitiveness of the galvanometer, 
the magnet is also used to adjust the needle to the zero in its 
position of rest by turning it slightly in one direction or the 
other. 

The shunts shown to the right of the galvanometer are also 
for the purpose of diminishing its sensibility by shunting definite 
known fractions of the main current past the galvanometer when 
the plug is inserted in the desired hole. 


538 


ELECTRICAL ENGINEERING TESTING 


If at any time the galvanometer needle should become in¬ 
sensitive and sluggish, it may be due to one of several causes, 
namely— 

(a) That the needle has become demagnetized. This can be 
remedied by withdrawing and re-magnetizing it with an ordinary 



General arrangement showing all connections. 

Fig. 222. 


horse-shoe magnet, care being taken that this is done in the same 
direction as before. 

(b) That some dirt has found its way into the jewel. This may 
be removed with a piece of soft wood cut to a fine point. 

(c) That the jewel or the needle point is injured. In this case 
the slide should be removed and sent with the needle and pointer 





































































ELECTRICAL ENGINEERING TESTING 


539 


to the makers for repair. This will probably have occurred 
either through the whole instrument having received a blow 
when the lid is open and the jewel resting on the needle point, 
or through the brass spring in the lid of the box being bent so 
that it no longer presses on the lifter when the lid is closed, and 
the needle has consequently been resting on the point while the 
box has been carried about. 

The remainder of the box consists of the two-way plug switch 
on the left of the galvanometer, by means of which either a known 
standard or unknown high-resistance can be separately inserted 
in series with the galvanometer. 

The spring tapping key seen on the left-hand lower corner of 
the box is for closing the galvanometer circuit when ordinary 
resistance other than that of insulation is being measured. 

The two cireular dials, marked tens and units, form the ad¬ 
justable resistance-arm of the Wheatstone Bridge arrangement, 
and consist of two sets of 9 coils each, totalling 99 ohms when 
both jjlugs are in the “9 ” holes, 0 when both plugs are in the 
“ 0 ” holes, and infinity when both plugs are out altogether. 

A double set of proportional coils or resistances, each consisting 
of 10, 100, and 1000 ohms coils, completes the bridge. These 
are connected to the row of blocks seen at the bottom of Fig. 222. 

The terminals shown are for connecting the battery and un¬ 
known resistance to. 


Evershed “Megger” and “Bridge-Megger” 

Testing Sets. 

The “ Megger ” Insulation Set. —The general principle under¬ 
lying the construction, as well as the internal connections, of 
this set will be seen by a reference to Fig. 223. As seen, the 
instrument is a combination of a magneto-generator on the right- 
hand side, with the ohmmeter portion on the left, in a somewhat 
unique form of magnetic circuit, common to both and consisting 
of two pairs of field poles braced by strong bar magnets NR, NR, 
and forming two bi-polar field magnets in series. In the right- 


540 


ELECTRICAL ENGINEERING TESTING 


hand one, and rotated by a folding handle and spur gearing Z), 
is the armature of the generator with its brush gear JI, and 
terminal bars marked and —. In the left-hand field is the 
current coil A, pressure coil P, and compensating coil G of the 
ohmmeter, connected to resistances Q and a “guard plate” 
and the only two external terminals L and E (marked Line and 



Earth in Fig. 224), which shows the general view of this set ready 
for use, with one end of the carrying strap detached from its 
spring cleat, the scale lid lifted and driving handle unfolded 
ready for use. 

The arrangement and connections of the moving coil system 
of the ohmmeter of this set are shown in Fig. 229, and that of 
the generator armature in Fig. 231, the coils of which are 
numbered consecutively in ordef of their winding, No. 1 being 



















































































ELEGTETGAL ENGINEEIilNG TESTING 


541 


next to the core. The generator of this set may be either of the 
variable or constant-pressure type. 

The “ Bridge-Megger ” Testing Set is available for use both 
as an insulation testing set and as a specialized Wheatstone 



Fig. 224. 


bridge. It differs from the above set in outward appearance 
only by the addition of two pairs of terminals at the left hand 
end, and of two switches near the top right-hand corner, as will 



Fig. 225. 


be seen by Fig. 225, showing a plan photograph of the box 

containing the ohmmeter and generator. 

One of the two switches is a Eatio Switch for varying the 

proportion of the two ratio arms, when the instrument is used as 
a Wheatstone bridge, so as to make the unknown resistance {X) 






























542 


ELECTRICAL ENGINEERING TESTING 


under test either eqtial to, or y^^h, or yj^^th of that of the standard 
resistance box R, thus providing a wide range of measurement 
which can be again increased by merely interchanging positions 
of the standard R and unknown resistances X relatively to the 
two pairs of terminals at the end (as seen in Figs. 44 and 45), 



which gives the unknown [X) now in terms of 7^ X 10 (or X 100) 
according to the ratio employed. 

The other, or two-way change-over switch, when set to “ Megger,’’' 
prepares the instrument for measuring large metallic resistance 


Fig. 226 . 


(vide p. 118), though principally insulation resistance, by coupling 
the two windings of the constant-pressure generator in series 
and making the two front terminals, marked Line and Eao'th^ the 
only two available for connection. 

When set to Bridge," the instrument is converted for Wheat¬ 
stone bridge work—the two windings of the generator being 
now coupled in parallel in order to increase the current obtain¬ 
able from it; the ohmmeter part being changed into a galva¬ 
nometer for the bridge; and the arms of the bridge being 
switched into their appropriate places in circuit and connected 
to the only available terminals (namely, the two pairs marked 
R and X at the end) for use now on the instrument. 







ELEGTRIGAL ENGINEERING TESTING 


543 


The variable standard direct-reading resistance box (R) for 
use in bridge measurements is shown in plan, Fig. 226, and is of 


O 

1 . 

3 

(/) 

U) 

to 

0.0 


c 

0) o 


L. 

3 

o 


’c <D 
^ CJJ 

ta 

X 

Ul 



(M 

(N 

d 

i-t 

Ph 



rn A 




u 




4-> L. 

eo 1) 
"in "O 
<U C 
OC 3 


the sliding-contact type, operated by turning ebonite handles. 
The figure appearing for each position of any handle is the resist¬ 
ance of that dial, and the total showm in Fig. 226 is 8,306 ohms. 
The complete internal connections of a “ Bridge-Megger ” set' 






































544 


ELECTRICAL ENGINEERING TESTING 


with change-over switch set to “Bridge” are given in Fig. 327, 
while tlie connections forming the usual bridge circuits are 
depicted in Fig. 228. 

The connections of the moving-coil systems are shown sepa¬ 
rately for the “Megger” set, Fig. 229—and. “Bridge-Megger” 
set. Fig. 230—while the armature connections of the generator 
are shown for these sets respectively in Figs. 231 and 232. 



The Constant Pressure Generator for the “ Bridge-Megger ” 
set is shown in Fig. 233, the permanent bar magnets N/S of Fig. 
223 being removed for clearness. 

The free-wheel attachment prevents the armature and gearing 
being damaged by any sudden stopping of the folding handle, 
and permits the armature to be driven in one direction only. 
Between the armature and gearing is interposed a centrifugal 
friction clutch comprising a drum driven by gearing, on which 
two arms, attached to the armature and fitted with pads at their 




















645 


BLBCTUICAL ENGlNimniNG TESTWG 


ends, are urged by springs. When the driving handle is turned 
above slipping speed, the speed of the armature, and hence its 
E.M.F., is extremely constant—varying as little as 1 part in 1000. 

The Index Adjuster, fitted to the latest constant-pressure 
sets, consists of a small piece of soft iron rod, mounted parallel 



Fig. 229. 



Fig. 230. 


llA. A 



Fig. 231. 



Fig 232. 


to the line joining the centres of the field poles and close to the 
moving coil C, Fig. 223. It is capable of being moved sideways in 
one direction or the other (relatively to 0) by means of a knob. 

The rod becoming magnetized inductively by the polar field 
causes some of its field to pass through the coil C, and hence, if 
the rod is moved, its field also moves slightly, which at the 
“infinity” position of the moving systems causes a slight deflec- 

N N 




















































































546 


ELECTRICAL ENGINEERING TESTING 


Pole Piece 





Constant Speed 
Clutch 
Free Wheel 

Folding Handl 


Fig. 233. 


Armature 
Brush Holder 
Commutator 
Roller Bearing 


tion of the pointer one way or the other, facilitating accurate 
adjustment to infinity. 

The adjustment does not affect the law of the instrument, 
which is altered only by the centre coil P, 


Standard Direct-Reading Electric Balance. 

Fig. 235 shows the general appearance (with glass cover 
removed), and Fig. 234 a part symbolical elevation of Lord 
Kelvin’s centi-ampere balance. 



]A 

]-& 

]C 


/ 


a> cv 


Fig. 234. 


A> L I ^ ,,W 

C' [- ! 


(1) The instrument is founded on the principle of action of the 
mutual forces, discovered by Ampere, between movable and 




































Fig. 235 



























































































































































548 


ELEGTRIGAL ENGINEERING TESTING 


fixed portions of an electric circuit. The shape chosen for the 
mutually influencing portions is circular, and each such part will 
be called for brevity an ampere ring, whether it consists of only 
one turn or of any number of turns of the conductor. 

(2) In this balance, each movable ring, R and R', is actuated 
by two fixed rings, AC and A’C '—all three approximately 
horizontal. There are two such groups of three rings—two 
movable rings attached to the two ends of a horizontal balance 
arm pulled, one of them up and the other down, by a pair of 
fixed rings in its neighbourhood. The current is in opposite 
directions through the two movable rings to practically annul 
disturbance due to horziontal components of terrestrial or local 
magnetic forces. 

(3) The balance arm is supported by two trunnions, each hung 
by an elastic ligament of fine wire fj through which the current 
passes into and out of the circuit of the movable rings. 

(4) The mid-range position of each movable ring is in the 
horizontal plane nearly midway between the two fixed rings 
which act on it. 

(5) The current goes in opposite directions through the two 
fixed rings, so that the movable ring is attracted by one of the 
fixed rings and repelled by the other. The position of the 
movable ring, equi-distant from the two fixed rings, is a position 
of minimum force, and the sighted position, for the sake of 
stability, is above it at one end of the beam and below it at the 
other, in each case being nearer to the repelling than to the 
attracting ring by such an amount as to give about per cent, 
more than the minimum force. 

(6) The balancing is performed by means of a weight which 
slides on an approximately horizontal graduated arm attached to 
the balance; and there is a trough fixed on the right-hand 
end of the balance, into which a proper counterpoise weight W 
is placed, according to the particular one of the sliding weights 
in use at any time (sect. 9 below). For the fine adjustment of 
the zero a small metal flag is provided, as in an ordinary chemical 
balance. This flag is actuated by a fork having a handle below 
the case outside, as shown at the bottom of Fig. 235. To set the 
zero the left-hand weight is placed with its pointer at the zero of 
the scale, and the flag is turned to one side or the other until it 


ELECTRIGAL ENGINEERING TESTING 


549 


is found that, with no current going through the rings, the 
balance rests in its sighted position. 

(7) To measure a current the weight is slipped along the scale 
until the balance rests in its sighted position. The strength of 
the current is then read off approximately on the fixed scale (called 
the inspectional scale), with aid of the finely divided scale for 
more minute accuracy, according to the explanations given in 
sect. 11 below. Each number on the inspectional scale is twice 
the square root of the corresponding number on the fine scale of 
equal divisions. 

(8) The slipping of the weight into its proper position is 
performed by means of a self-releasing pendant, hanging from a 
hook carried by a sliding platform, which is pulled in the two 
directions by two silk threads passing through holes to the out¬ 
side of the glass case. 

(9) Four pairs of weights (sliding and counterpoise), of which 
the sledge or carriage and its counterpoise constitute the first pair, 
are supplied with the instrument. These weights are adjusted 
in the ratios of 1:4:16: 64, so that each pair gives a round 
number of amperes, or half-amperes, or quarter-amperes, or of 
decimal sub-divisions or multiples of these magnitudes of current, 
on the inspectional scale. 

(10) The useful range of each instrument is from 1 to 100 of 
the smallest current for which its sensibility suffices. The range 
of this instrument is from 1 to 100 ceuti-amperes. The following 
table shows the value per division of the inspectional scale 
corresponding to each of the four pairs of weights—• 

Centi-amperes 
\ per division. 

First pair of Weights. . . . 0*25 

Second „ ..... 0’50 

Third „ .I'O 

Fourth ,, . . . . . 2‘0 

(11) The fixed inspectional scale shows, approximately enough 
for most purposes, the strength of the current; the notches in 
the top of the aluminium scale show the precise position of the 
weight corresponding to each of the numbered divisions on the 
fixed scale, which practically annuls error of parallax duo to the 


550 


ELECTRICAL ENGINEERING TESTING 


position of the eye. When the pointer is not exactly below one 
of the notches corresponding to integral divisions of the 
inspectional scale, the proportion of the space on each side to the 
space between two divisions may be estimated inspectionally 
with accuracy enough for almost all practical purposes. Thus we 
may readily read off 34*2 or 34*7 by estimation with little 
chance of being wrong by 1 in the decimal place. But when 
the utmost accuracy is required, the reading on the fine scale 
of equal divisions must be taken, and the strength of current 
calculated by aid of the table of double square roots given at 
the end of this book. Thus, for example, if the reading is 292, 
we find 34*18, or say 34*2, as the true scale reading for strength 
of current; or, again, if the balancing position of the pointer 
be 301 on the fine scale, we find 34*70 as the true reading of the 
inspectional scale. 

(12) The centi-ampere balance, with a thermometer to test the 
temperature of its ampere rings, and with platinoid resistances 
up to 1600 ohms, serves to measure potentials of from 10 volts 
to 400 volts, and up to 2000 volts with specially constructed high 
resistances. 

Table X. 


Constant of the Centi-ampere Balance when used as A 

y cltmeter. 


Weight used. 

Resistance in 
circuit. 1 

Volts per division 
of fixed scale. 

First Pair of Weights . 

400 

10 

n >» ••• 

800 

20 

>1 >> ••• ••• 

1200 

8-0 

>> ^ ••• ... 

ICOO 

4-0 


1 Including resistance of the instrument, which is about 50 ohms. 

If the second pair of weights is used, the constants will be 
double of those noted above. 

(13) Instructions for the Adjustment of the Standard Balances. 
—The instrument should be levelled in accordance with the 
indications of the attached spirit level, by means of the levelling 
screws on which the sole-plate of the instrument stands. 

(14) In this centi-ampere balance, the beam can be lifted off 
its supporting ligaments by turning a handle attached to a shaft 








ELEGTRIGAL ENGINEERING TESTING 


551 


passing under the sole-plate of the instrument. This shaft 
carries an eccentric, on the edge of which rests the lower end of 
a vertical rod, which is fixed at its upper end to a tripod lifter. 
W^hen the instrument is to be packed for carriage, or when it is 
to be removed by hand from place to place, the lifter should be 
raised j but when it is fixed up for regular use, it is advisable to 
keep the beam always hanging on the ligaments. 

(15) The carriage is fitted with an index to point to the 
movable scale, and is intended to remain always on the rail. 
One or other of the weights is to be placed on the carriage 
in such a way that the small hole and slot in the weight pass 
over the conical pins. The weights are moved by means of a 
slider, which slides on a rail fixed to the sole-plate of the 
instrument, and carries a pendant with a vertical arm intended 
to pass up through the rectangular recess in the front of the 
weight and carriage. The slider and weight are shown in position 
in the figures. The slider is moved by silk cords, which pass 
out at the ends of the glass case. When the cords are not being 
pulled for shifting the weight, their ends should be left free so 
that the pendant may hang clear of the weight. When a weight 
is to be placed on or removed from the carriage, tli(e slider should 
be drawn forward at the top until it is clear of the weight, and 
then pushed to one side until the weight is adjusted, when it 
may be replaced in position in a similar manner. 

(16) Cylindrical counterpoise weights with a cross-bar passed 
through them are supplied for the purpose of balancing the slid¬ 
ing weights when they are placed at the zero of the scale. The 
sliding weight should be placed so that the index of the carriage 
points to the zero of the scale, and the proper counterpoise 
weight should be placed in the trough, fixed to the right-hand 
end of the beam, with its cross-bar passing through the hole in 
the bottom of the trough. The flag which is attached to the 
cross-trunnion of the beam should then be turned by means of 
the handle projecting from under the sole-plate, until the index 
on the end of the movable scale points to the middle one of the 
five black lines on the fixed scale opposite to it. Care must be 
taken when making this adjustment that the fork which moves the 
flag is not left in contact vnth it, as this would impede the free 
swing of the beam. The fork should be turned back a little after 


552 


ELECTRICAL ENGINEERING TESTING 


each adjustment of the flag, and, when the flag is being adjusted, 
it is better to watch the flag itself, and make successive small 
adjustments until the beam stands at zero, than to make succes¬ 
sive trials by pushing round the handle while watching the 
position of the index. 

If the ligament has stretched since the instrument was stand¬ 
ardized, the index at one end of the movable scale will be found 
to be below the middle line on its vertical scale, when the index 
at the other end is correctly pointing to the zero position. The 
error so introduced would be a small one, but it may be easily 
put right by slightly loosening the screws fixing the pillared 
frame, which supports the movable beam, to the base plate, and 
raising it by slipping one or two thicknesses of paper below it 
until the indices simultaneously point to their zero position. 

(17) A lens is supplied with each instrument for facilitating 
accurate observation, either when reading the position of the 
weight or when adjusting the zero. 

(18) The vibrations of the beam may be checked so as to 
facilitate reading by bringing the pendant, which moves the 
weight, lightly into contact with it, in such a way as to give a 
little friction without moving the weights. 

(19) In using the centi-ampere balance as a voltmeter when 
great accuracy is required, care must be taken that the effect of 
change of temperature in changing the resistance of the coils of 
the instrument, and of the external resistance coils, is allowed 
for; and in this use of the instrument it is advisable to employ 
currents such as can be measured by the lightest weight on the 
beam. When the instrument is to be used as a voltmeter, four 
resistances are provided, three of which are each 400 ohms, and 
the fourth is less than 400 ohms by the resistance of the coils of 
the instrument at a certain specified temperature. The smallest 
resistance is intended to be included by itself in the circuit when 
the lowest potentials are being measured, and in series with one 
or more of the others when the potential is so high as to give a 
stronger current than can be measured with the lightest weight 
on the beam. The correction for temperature is, for the copper 
coils of the balance, about 0‘38 per cent, per degree Centigrade, 
and for the platinoid resistances, about 0‘024 per cent, per degree 
Centigrade. 


ELECTlilCAL ENGINEERING TESTING 


^ •'I 

oo3 


Anti-Inductive Resistance for the Kelvin 
Standard Electric Balances. 

When a balance of the above type, such as, for instance, the 
centi-am^yeo'e or coonposite instrument, is to be used as a voltmeter, 
four resistances are provided, three of which are each 400 ohms, 
and the fourth is less than 400 ohms by the resistance of the 
coils of the balance at a certain specified temperature. These 
resistances are doubly wound so as to be non-inductive, and are 
made of platinoid wire, wound on suitable frames, so as to pre- 



Fig.236. 


sent a maximum amount of cooling surface. The frames are 
enclosed in a box with apertures at the top to allow of the warm 
air getting out. Fig. 236 represents such a box for use with a 
Kelvin standard balance. The smallest resistance is intended to 
be included by itself in the circuit when the lowest potentials 
are being measured, and in series with one or more of the others, 
when the potential is so high as to give a stronger current than 
can be measured with the highest weight on the beam. The 
correction for temperature is, for these anti-inductive platinoid 
resistances, about 0‘024% per degree Centigrade. 



554 


ELECTRICAL ENGINEERING TESTING 


Composite Balance. 

(1) This instrument is similar in form to the centi- and deci¬ 
ampere balances, but the pair of fixed coils at one end of the 
beam are made of a rope of insulated wires similar to that used 
for the coils of the hekto-ampere balance. Separate electrodes are 
provided for the rope coils, and for the fine wire coils. A switch 
which allows the movable coils either to be included in the 
circuit by themselves or in series with the fixed fine wire coils is 
attached to the under side of the sole-plate of the instrument. 
When the handle of the switch is turned to “ Watt,’’ the mov¬ 
able coils alone are in the circuit; but when the handle is 
turned to “ Yolt,” both the movable and the fixed fine wire coils 
are in the circuit. 

(2) The composite balance (Fig. 237) can be used as a hekto- 
ampere balance, or as a Wattmeter, or as a voltmeter, by follow¬ 
ing the instructions given below. 

To enable the composite balance to be used as a direct-reading 
Wattmeter or voltmeter, a separate anti-inductive resistance of 
platinoid wire, subdivided into four coils, is usually supplied. 
The first coil is equal to the resistance of the fixed fine wire coils, 
and is intended to be included in the circuit of the movable 
coils when the instrument is used as a Wattmeter. The second 
coil is arranged to make up 200 ohms with the resistance of the 
fine wire movable and fixed coils. The third coil is 200 ohms 
and the fourth is 400 ohms. 

It is not advisable that the current through these resistances 
should be allowed to exceed 0*5 ampere. 

Instructions for the use of the Composite Balance. 

(3) The balance should be levelled and the stop screws turned 
back out of contact with the cross trunnion and the front plate 
of the beam so as to leave it free to oscillate. 

(4) To use the instrument as a centi-armpere meter or as a 
voltmeter the switch is turned to ‘‘ Yolt,” and one or other of 
the weights marked FlFi, VW^, VW^, used. The current flowing 
through the instrument is then to be calculated from the constant 
given in the certificate sent with the instrument. 



Fig. 237. 
































































































































550 


ELECTRICAL ENGINEERING TESTING 


Constant of Composite Balance when used as a Centi- 

AMPERE Balance. 


Weight used. 

Sledge + FITj, 
,, + VJV^. 

+ VW„ 


Ceriti-amperes per Division 
of Fixed Scale. 

. 0-5 

. 1-0 

. 2-0 


The volts on the terminals are calculated from the current in 
amperes and the resistance in ohms (including the anti-inductive 
resistance, if any) in circuit. If V be volts, 0 current, and R 
resistance, 

V=CR. 


The anti-inductive resistance is arranged so that the instru¬ 
ment reads a round number of volts per division. 


Table XI. 

Constant of Composite Balance when used as A Voltmeter. 


Weight used. 

Sledge + VW, 

JJ 5) 


Resistance 
in Circuit.l 

200 

400 

800 


Volts per Division 
of Fixed Scale. 

. 10 

. 2*0 

. 4-0 


1 Including the resistance of the instrument, which is about 30 ohms. 


If the second pair of weights (Sledge-f VW^ be used the con¬ 
stants will be double of those noted above. 

' (5) To use the instrument as a hekto-ampere meter the switch 

is turned to “ Watt ” and the thick wire coils inserted in the 
current circuit in such a way that the right-hand end of the 
beam is repelled up. Either the sledge alone or the weight 
marked TFTF is to be used in this case. A measured current is 
then passed through the suspended coils, and the constants given 
in the certificate for the balance used in this way are calculated 
on the assumption that this current is, as there stated, 0*25 
ampere, but any other current which is convenient in the cir¬ 
cumstances may be used. The current through the suspended 
coils maybe measured by means of the instrument itself arranged 
for the measurement of volts. This may be done by first mea- 


ELECTRICAL ENGINEERING TESTING 


557 


suring the current which the difference of potential between the 
supply conductors of an electrical installation, or between the 
poles of a battery, causes to flow through the coils of the instru¬ 
ment and its external resistance, and then turning the switch to 
“ Watt, and at the same time introducing a resistance into the 
circuit equal to the resistance of the fixed coils. 


Constant of Composite Balance when used as a Hekto- 

AMPERE Balance. 


Weight used. 

Sledge + 

„ + W]V,, 

„ + irir3, 


Amperes per Division 
of Movable Scale.i 

0-250 
. 0-500 

1-000 


I With 0‘25 ampere through movable coils. 


N.B.—The constants vary inversely as the current through the 
fine wire coils. 

(6) To use the instrument as a Wattmeter, one terminal of the 
fine wire coils is joined to one end of the anti-inductive resist¬ 
ance and the other terminal to one of the leads; the other end 
of the resistance being joined to the other lead. The thick wire 
coils are inserted in the main circuit as described in sect. 5 above. 
With the instrument thus joined up, the current through the 
suspended coils and the E.M.F. between the leads may be obtained 
by the operations described in sect. 4 above, since the presence of 
the thick wire coil in the circuit causes no appreciable error: 
or the E.M.F. may be taken from the electrostatic voltmeter 
used on the circuit, and from its indications the current in the 
suspended coil circuit calculated. The watts are then to be 
calculated from the E.M.F. on the leads and the current through 
the thick wire coils by the formula 

W=VC = cGR, 

where c is the current in the suspended coil circuit, G the current 
in the thick wire coils, and R the resistance in the circuit. 

The weights sent out with the instrument are arranged to give 
a round number of Watts per division of the scale with a known 
anti-inductive resistance in series with the fine wire movable 
coils. 


ELECTRICAL ENGINEERING TESTING 



Table XII. 

Constant of Composite Balance when used as a Wattmeter. 


Weight used. 

Resistance In Circuit 
with Movable Coils.i 

Watts per Division 
of Movable Scale. 

Sledge 

+ TTlFi, 

. 200 


. 12-5 

5) 

if 

. 400 


. 25-0 

>> 

if 

. 800 


. 500 

?> 

+ TFlTg, 

. 200 


. 25-0 


if 

. 400 


. 50-0 


if 

. 800 


. 100-0 


+ WW^, 

. 200 


. 50-0 

>5 

if 

. 400 


. 100-0 

5> 

if 

. 800 


. 200-0 


1 Including resistance of moyaWe coils, which is about 12 ohms. 


Adjustable Magneto-static Current Meter. 

(1) The magneto-static current meter (Fig. 238) consists essen¬ 
tially of a small steel magnet or system of magnets suspended in 
the centre of a uniform field of force due to two coils, each having 
one or more turns of copper ribbon or wire, and also under the 
directive influence of two systems of powerful steel magnets. 

(2) The suspended system of magnets is attached to one end of 
a vertical shaft passing down centrally through an opening in the 
sole-plate of the instrument from an indicating needle, which is 
supported by a jewelled cap resting upon an iridium point. 

(3) The two systems of directive magnets are circular in form, 
and each ring is composed of two semicircular magnets placed in 
a brass cylindrical frame with their similar poles together. Each 
system is securely fixed to a circular brass frame, which fits on to 
the cylindrical case of the instrument in such a manner that the 
systems are capable ©f being turned round, together or separately, 
as explained below. 

(4) The instrument has a “ tangent scale,” which is adjusted 
in its position before the instrument is sent out, so that the needle 
indicates equal differences of readings for equal differences of 
current. The scale consists of a hundred divisions, and for most 
purposes it is convenient to set the field magnets in such a position 





ELECTBIGAL ENGINEEBING TESTING 


550 

that the needle points to 0, and to use the scale from that point 
upwards towards 100. Sometimes, however, it may be found 
convenient to measure currents, whose direction is being occasion¬ 
ally reversed, without being at the trouble of reversing the 
electrodes in the contact clip; in that case the zero should be set 
to the division 50 at the middle of the scale, and readings taken 
on each side of it. It must be remembered that when the point 
taken as zero is changed, the constant, by which the indications 
of the instrument have to be multiplied to give the current in 
amperes, is changed in proportion to the cosine of the angle 
between the zero point and the middle of the scale; and as this 



Fig. 238 . 


angle is 60“, the constant with the zero at 50 on the scale is 
exactly double the constant with the zero at 0 on the scale. 

(5) The instrument is provided with a “ lifter,” which serves to 
raise the needle off the iridium point when it is being moved 
about from place to place. This lifter is in the form of a ring 
placed below the needle, and may be raised or lowered by turning 
the handle attached to an eccentric passing through the side of the 
instrument on a level with the scale. It also serves as a checker, 
by bringing it lightly into contact with the pointer, so as to stop 
its vibrations. 

(6) The instrument has an advantage, important for some 
practical purposes, of being available as an accurate direct-reading 








ELECTRICAL ELfGlNLERlNG TESTING 


5r,o 

current meter, through a continuous range of from 1 to 100 times 
its smallest current, which mny be anything from half a milli- 
ampere to 4 amps., according to the number of turns in the coils 
supplied with the instrument. It is not, however, available as an 
alternate current instrument, and it must be remembered that 
the magnetism of the steel directing magnet does not remain 
absolutely constant. With good quality of steel, a proper pre¬ 
liminary ageing of the magnet (by heating it several times in 
boiling water and cooling it again, and subjecting it to somewhat 
varied rough usage) brings it to a condition in which its magnetism 
is found to remain exceedingly nearly constant month after month 
and year after year. Still, it should never be relied upon as 
absolutely constant, and for accurate laboratory work it is there¬ 
fore necessary to occasionally standardize it. 

(7) Another advantage which the instrument has is that, when 
a standard instrument is available, its constant is capable of being 
varied to any desired value down to one-tenth of that which it has 
with its directive magnets in their strongest position. Thus if 
the constant should be 3 amps, per division of the scale, with the 
similar poles of the magnets coinciding, it may be adjusted to any 
value down to 0'3 amp. per division. 

(8) Instructions for Use of the Magneto-static Current 
Meters. — The instrument should be levelled, in accordance 
with the attached spirit-level, by means of the levelling 
screws. 

(9) To Adjust the Pointer to Zero. — (a) Loosen the two lower 
milled-headed screws clamping the magnet frame, and turn the 
frame round till the pointer stands at zero, {b) Reclamp the 
frame by tightening the two screws. 

(10) Adjustment of the Scale. —The scale, as stated above (sect. 
4), is firmly clamped in its place before sending the instrument 
out, and this position is marked by two lines on the outside of the 
case, one horizontal and the other vertical, just below the 0 of 
the scale. The horizontal line is engraved below the movable 
top of the instrument, and the vertical one on the side of the case. 
Should the top of the instrument have been inadvertently moved, 
and the scale thus put out of adjustment, it may be set right by 
slightly loosening the two slotted screws and turning the top 
round till the extremities of the two lines coincide. 





ELEGTRIOAL ENGINEERING TESTING 561 

(11) If the needle should by accident be slightly bent,^ and so 
render a new adjustment of the scale necessary, this may readily be 
made in the following manner :—Set the zero, by the field magnets, 
to the division 50 at the middle of the scale, then join the instru¬ 
ment in series with another current instrument of convenient 
form, and pass a current through both sufficient to give a deflec¬ 
tion of about 40 divisions on the magneto-static instrument ^ 
reverse the current on the magneto-static instrument only, and 
set the scale so that equal deflections, read in divisions, are given 
on each side of the zero for equal currents, as indicated on the 
auxiliary instrument. The zero must, of course, be reset by the 
magnets every time the scale is moved. When the scale has been 
adjusted to this position, firmly clamp the top of the instrument 
by the two slotted screws, and again mark the position of the 
horizontal line on the outside of the case. 

(12) Adjustment of Constant .—The constant may be quickly 
varied as follows :—Join the instrument in series with any reliable 
current instrument of known accuracy, such as the deci-ampere 
balance, and pass a convenient current through both instruments, 
observing the readings. Break the current, loosen the two upper 
pairs of milled-headed screws, and turn the top system of magnets 
relatively to the lower, so that the similar poles of the two 
systems are brought closer together or moved further apart, 
according as it is desired to make the instrument respectively less 
or more sensitive. Reclamp the screws and adjust the zero as 
described in sect. 10. Again make the current, and note the 
reading on the two instruments. The desired reading on the 
magneto-static may be obtained quickly after one or two approxi¬ 
mations, care being always taken to readjust the zero after each 
movement of the top magnets. 

(13) When convenient it is always best to standardize the 
instrument in the place where it is to be used ; but when it is 
intended to move it from place to place, it should be standardized 
in such a position that when the needle is pointing to zero it is in 
a direction approximately east and west. 

^ If it is bent so largely as to be perceptible to the eye, it ought to be 
straightened by hand as nearly as may be. 


562 


ELEGTRIGAL ENGINEERING TESTING 


Electrostatic Voltmeters. 

These voltmeters have the great advantage of being available 
as accurate measurers of potential on direct and alternating 
systems, and, being electrostatic, they use no current, and conse¬ 
quently require no temperature correction. They are therefore 
free from the causes of error so prevalent in instruments of the 
electro-magnetic type, whose accuracy is impaired by variations 
of temperature, and which when used on alternating systems are 
alTected by errors due to self-indaction ranging with the period 
of alternation. 

The instruments are made on the principle of an air condenser, 
having one of its parts movable about an-axis, so as to increase 
or diminish the capacity. The condenser is enclosed in a metal 
case, for the double purpose of protecting the movable part from 
air currents, and from the disturbing influence of any electrified 
body, other than the fixed portion, differing from it in potential. 
In these instruments, the fixed portions consist of two sets of 
quadrant-shaped cells in metallic connection with each other, and 
formed by a number of parallel brass plates. These cells are 
fixed by an insulating support to the case of the instrument, and 
a terminal passes from them to an insulated binding screw on 
the outside of the case. 

The movable portion in all the instruments is in metallic con¬ 
nection with the surrounding case. In the multicellular volt¬ 
meters this connection is made through the suspending wire. 
The movable portion carries the pointer, which indicates by direct 
readings the difference of potential between the two parts of the 
condenser. 

The action of the instrument, shortly stated, is as follows:— 
When the fixed and movable plates are connected respectively to 
two points of an electric circuit, between which there exists a 
difference of potential, the movable plate tends to move so as to 
augment the electrostatic capacity of the instrument, and the 
magnitude of the force concerned in any case is proportional to 
the square of the difference of potential by which it is produced. 
In the multicellular voltmeters this force of attraction is balanced 
by the torsion of the suspending wire. 




ELECTRICAL ENGINEERING TESTING 


563 


The Kelvin Multicellular Electrostatic 

Voltmeter. 

Tlie arrangement of the parts of this instrument is shown in 
Figs. 239, 240, and 241, These figures apply to an early form of the 



Fig. 239. 


voltmeter, and differ in two matters of detail from the voltmeter 
as now made. For simplicity in manufacture the cells are now 
made with straight backs, and the plates looked at in plan are, 
therefore, triangular instead of square, as shown in Fig. 241. A 
coach-spring has now been interposed between the suspending 
wire and the spindle carrying the vanes, as explained below. 











5«4 


ELECTRICAL ENGINEERING TESTING 


The insulated cells are formed of triangular brass plates fixed 
into saw cuts in a brass back piece so as to be equal distances 
apart and accurately parallel to each other. Two sets of those 
cells C are fixed relatively to each other, as shown in Fig. 240, by 
a vulcanite support to the sole-plate, so that their plates are 



horizontal, and are completely enclosed within the brass cylin¬ 
drical case of the instrument. 

On the top of this cylin ler is a shallow horizontal circular 
scale-box containing the scale of the instrument, and having a 
glass cover, which serves to protect from currents of air the 
movable indicator /, and the scale and interior parts from dust. 
























































































































































ELEGTRIGAL ENGINEERING TESTING 


565 


For the movable part a number of vanes, V, similar in form to 
those of the quadrant electrometer are used. These vanes are 
placed parallel to each other on a spindle with distant pieces 
between them. The top end of this spindle passes through a 
small hole in the sole-plate of the instrument, which forms the 
bottom of the scale-box, and is attached to a small coach-spring, 
which in turn is secured to one end of a fine iridio-platinum wire 
suspended from a torsion head at the top of a vertical brass tube. 



The torsion head may be turned by means of a forked key pro¬ 
vided for the purpose, and is clamped, to protect it from accidental 
displacement, by a cap which screws on to the end of the tube. 
The coach-spring has sufficient resilience to allow the spindle to 
touch a guard stop, and so saves the suspension from injury in 
event of the instrument being roughly set down. 

Two vertical brass repelling plates, which also act as guard 
plates to prevent the movable part from turning beyond its 
prescribed limits, are fixed to the bottom of the sole-plate. These 

















566 


ELECTRICAL ENGINEERING TESTING 


two plates carry a guide plate, G, with a circular opening in it, 
through which the lower end of the spindle passes. A little brass 
disc, or head, D, is attached to the end of the spindle, sufficiently 
large to prevent its passing back through the hole in the guide 
plate. Thus the movable part is effectually secured from swinging 
about so as to be injured, and by no possibility can it come into 
contact with the insulated quadrants. When the instrument is 
level the spindle hangs free by the suspending wire, so that the 



Fig. 242a. 


Fig. 2 m . 


vanes are horizontal, and each is in a plane exactly midway 
between those of two contiguous condenser plates. 

An aluminium needle attached to the top of the spindle indicates, 
on the horizontal circular scale fixed to the upper side of the sole- 
plate, the difference of potential between the movable and fixed 
portions of the condenser by direct readings in volts. 

Engine-room Pattern Multicellular .—The description of the 
instrument given above refers to the horizontal scale or laboratory 
pattern. In the new engine-room pattern (Fig. 242 a and 6), the 







































































































JELEGTlilGAL ENGINEERING TESTING 


567 


parts are in every way similar, but the instrument has a vertical 
scale. A vane attached to the spindle turns in an oil dash-pot 
and gives the instrument a dead-beat action. 

Portability. — A small thumb-screw is placed in the centre of 
the base plate below the instrument, which can be screwed in so 
as to lift the weight of the spindle and vanes from the suspending 
wire and clamp the disc on the end of the spindle against the 
guide plate. A lifter or checker is also provided similar to 
that used in the magneto-static instruments. 

A switch is attached to the insulated terminal of the instrument 
by which the voltmeter can be taken out of circuit when desired. 
The switch, after breaking circuit, puts the case and the in¬ 
sulated cells in metallic connection. 

Instructions for the Use of the Multicellular Electrostatic 
- Voltmeter. 

When received from the maker the indicator needle with 
attached vanes will be found supported by means of the thumb¬ 
screw below the instrument, and also by the circular lifter, 
or checker, turned up so that the weight of the needle and vanes 
is taken off the suspending wire. 

The scale is graduated to read directly in volts. 

To set the instrument up for use. — {a) Unscrew the thumb¬ 
screw, and turn down the checker, so that the needle swings 
clear; (5) level the instrument so that the spindle of the vanes 
passes down centrally through the intersection of the two black 
cross-lines on the sole-plate. 

To adjust the zero, if necessary. —Unscrew the cap on the top of 
the tube, remove the washer, turn the torsion head by means of 
the forked key until the pointer stands at 0 on the scale. Replace 
the washer and screw on the cap again. Before adjusting the 
zero turn the switch so that the insulated cells are in metallic 
connection with the case. 

Arrangement for portability. —When the instrument is to be re¬ 
moved from place to place, see that the needle is lifted by turning 
up the checker, and when it is packed for use as a portable in¬ 
strument, always screw up the thumb-screw as mentioned above. 

As aluminium is electro-positive to brass, the instrument reads 


568 


ELEGTRIGAL ENGINEERING TESTING 





about ^ of a volt too low when the positive pole of a battery or 
dynamo is attached to the upper or insulated terminal of the 
instrument; and about I volt too high if connected in the 
opposite direction. With alternating currents it is correct. 



Fig. 243 , 









































ELECTBICAL ENGINEERING TESTING 


5r<9 


Crompton D’Arsonval Galvanometer. 

A convenient form of sensitive galvanometer, designed for 
laboratory use, with a large range of adjustment, and made by 




d 

I—I 


Messrs. Crompton and Co., Chelmsford, is shown in Fig. 213, and 
the details of construction in Figs. 244 and 245. 

The moving part of the instrument is shown in Fig. 245. A 




















570 


ELECTRICAL ENGINEERING TESTING 


circular coil of wire hangs by a bifilar suFspension between the 
poles of a permanent magnet, an iron core being fixed in the 
centre. The suspension ligaments are of very thin copper strips, 
and are connected to the coil by means of a silver clip, which 
allows the coil to be easily disconnected. 



Fig. 245. 

The mirror is hung from the coil by fine aluminium hooks 
passing through holes pierced in the mirror, so that this is easily 
detached, and is not distorted by the setting of cement or the 
pressure of a clip. 











ELECTRICAL ENGINEERING TESTING 


571 


Fig. ‘i45 shows the construction of the bifilar suspension head. 
The ligaments are attached to two pins aa fixed in a disc, by- 
turning which the tensions of the two may be made equal. 
They pass over two pins bb placed on another disc, by turn¬ 
ing which the distance between the ligaments may be ad¬ 
justed, and the sensitiveness of the instrument increased or 
diminished. 

The whole head is raised or lowered by turning the milled edge 
G, and is rotated slowly by turning the worm spindle d. 

The two pillars by which the cover / is secured serve as the 
terminals of the instrument. 

Coils are made having different numbers of turns from 100 
upwards, and the sensitiveness of the instrument when adjusted 
to give a complete period of oscillation of from eight to ten 
seconds is nearly as follows— 


Table XIII. 


No. of TuriiS 

Resistance, includ- 

Deflection of beam in minutes of Arc 

on Coil. 

ing Ligaments. 

For 1 Micro-volt. 

For 1 Micro-ampere. 

100 

. 2 ohms 

6 

35 

300 

30 „ 

3-5 

105 

1000 

1000 ,, 

0-6 

350 


The coils can be fitted with small closed rings of copper, which 
damp their movements to any desired degree. 

Without these the coils are for practical purposes perfectly 
ballistic. 

An electric lamp used without a lens is the most convenient 
light for the above galvanometer, the filament being focussed on 
the screen by the mirror. This latter is large (25 m.m. diameter) 
and ground to a radius of one metre. 


Sensitive Portable Galvanometer. 

When a Wheatstone Bridge has to be used for outside work, 
other than in a test-room or laboratory, or when fairly delicate 
tests have to be made on the “ line,” a portable type of galvano¬ 
meter or detector which is as sensitive as possible must be 










572 


ELECTRICAL ENGINEERING TESTING 



available. One of the best forms of such an instrument is illus¬ 
trated in Fig. 246. It consists of a fairly flat-shaped coil of 

fine insulated wire, having a resist¬ 
ance usually of between 1000 and 
2000 ohms, placed on its side in a 
brass contain-case provided with a 
glass top and glass window in the 
side just opposite the scale. A 
magnetic needle, to which a long 
light pointer is attached, is pivoted 
between jewelled bearings in the 
middle of a flat rectangular brass 
tube which is capable of being 
slipped inside the similarly shaped 


Fig. 246. 


aperture in the coil of wire. 

' The pointer protrudes outside one end of the coil and moves 
over a suitable scale, part of which is seen to the left of Fig. 
246. A strip of mirror is let in under the scale and shows 
through an aperture in it, thus enabling errors due to parallax 
to be avoided. A needle clamper, actuated by a button on the 
edge of the case, enables the needle to be clamped during 
transport and damage to the pivoting thus .avoided. 

When no current flows the magnetic axes of coil and needle 
are perpendicular, when the pointer is at 0 at the middle of the 
scale. Then the effect of a current is to cause the needle to set 
itself parallel to the axis of the coil, so giving a deflection to one 
side or the other of zero. This form of instrument is a very 
sensitive one and very suitable for portable work with a Wheat¬ 
stone Bridge. 


Parr’s Direct-Reading Dynamometer 
Measuring Instruments. 

These instruments depend for their action on the mutual force 
of repulsion between two circuits or coils carrying either the 
same or different currents, one circuit being fixed and the other 
movable. The instruiiients, which have been devised and per¬ 
fected by the author, possess some very important properties 


















ELEGTEWAL ENGINEERING TESTING 571^ 

that it may be well to note here. They contain no iron what¬ 
ever and very few metal parts, consequently they measure 
.either the true watts, volts, or am'peres, as the case may be, in 
any alternating current circuit, and are quite independent of the 
periodicity of the circuit. They are of the switchboard type, 
direct-reading, and have extremely open scales, extending over 
Y^^ths of the circular dial. 



Fig. 247. 


Fig. 247 shows an ammeter for 84 amperes, the scale gradu¬ 
ations commencing at 3 amps, and continuing nearly uniformly 
to the end. Fig. 248 shows an internal view of this same 
7-inch ammeter. As seen it consists of two fixed coils and two 
moving coils, carried at the end of a horizontal arm capable of 
rotating on a vertical spindle pivoted in jewelled centres. This 
spindle has rigidly attached to it a horizontal arm, to the end of 
which is fixed a flexible metallic strip that passes almost once 
round a special pulley carried by a horizontal spindle moving in 
jewelled centres and carrying the pointer at the front end, a 






574 


ELEGTRIGAL ENGINEERING TESTING 


hair-spring, and balance-arm for the pointer. The moving coils 
and pointer are controlled by the hair-spring, the tension of 
which can be adjusted by a moving arm. Current is let into 
and out of the moving coils through non-spillable mercury cups. 
A damping vane and trough is added, also unspillable, by means 
of which the instruments are made dead beat to any desired 



Fig. 248. 


extent. The moving coils are clamped by a suitable arrange¬ 
ment during transport and seen to the right of Fig. 248. The 
moving coils are in contact with their respective fixed coils 
when the pointer is at 0 and no current flows. Repulsion 
ensues when a current is sent through the instrument, and 
according to whether it is an ammeter, voltmeter, or Wattmeter, 
so the pointer deflects and indicates directly the quantity to be 
measured. The instruments of course read equally accurately 














ELEGTRIOAL ENOINEERING TESTING 


575 


on direct-current circuits, and having such very wide, open and 
uniform scales, a 6-inch instrument can be read with certainty 
many yards away. 

Siemens Torsional Voltmeter. 

This instrument may be used either as an ammeter for very 
small currents or as a voltmeter with a very extended range, 



Fig. 24y. 


and provides a good example of the method of converting one 
into the other. 

It consists of two coils, wound with fine insulated wire, and of 
elongated oval section, with their axes collinear and horizontal. 
They are connected in series and wound so that the north pole 






57H 


ELEGTRTOAL ENGINEERING TESTING 


of one faces the south pole of the other, i. e. the two coils may 
be regarded as one coil with a gap in the centre. 

Between the coils is placed a horse-shoe or bell magnet 
suspended by a silk fibre. A spiral spring is attached to the 
magnet and to a torsion head at the top of the case, so that by 
turning the head, a twist is applied to the magnet, proportional, 
of course, to*the angle of turning of the torsion head. To the 
magnet is attached a pointer, for the zero position of which the 
magnetic axes of coils and magnet are at right angles. 

The magnet also carries an aluminium vane, moving between 
two brass cheeks which act as stops, the vane assisting in 
stopping the vibrations. 

On passing a current through the coils so as to induce the 
polarity indicated by the small Tetters ns, ns (Fig. 250), which 
represents a symbolical sectional plan of the coils G and needle NS 
in a horizontal plane passing through their centres, the magnet 
NS tends to turn counter-clockwise in the direction of the 
arrow, so that its magnetic axis would coincide with that of 
ns, ns. Then the angle through which the torsion head has 
to be turned (in a clockwise direction) in order to bring the 
magnet pointer back to zero measures the moment of the couple 
exerted by the coils on the magnet, i. e. the current flowing 
through those coils. 

The coils C together have a resistance of 100 ohms in the 
instrument illustrated, and it is wound with such a number of 
turns that when IT volts are placed across the terminals of the 
instrument itself, the torsion head makes one complete turn 
or 170 divisions to bring the magnet pointer back to zero. 
Hence 1 scale division = O’Ol volt. To make it read OT and 1 
volt per division, 900 and 9900 
ohms respectively must be put 
in series with the instrument, 
when the extremities of the com¬ 
bination will now form the volt¬ 
meter terminals. These extra 
anti-inductive resistances are con¬ 
tained in the receptacle to the 
left in Fig. 249, which is provided with a plug top for inserting 
these resistances at pleasure. They should be wound with a 









ELECTRICAL ENGINEERING TESTING 


577 


material having a high specific resistance and low temperature 
coefficient of variation of resistance for reasons already given. 

ihis instrument can he used as a low-reading ammeter, for 
since 1 division = 0*01 volt and the whole resistance of its coils 
= 100 ohms, . 1 division = ^^y^ = 0*0001 ampere. 

Adjustment of voltmeter. —If the magnet has been clamped 
for transport, release it by turning the milled-headed rod which 
passes through the edge of the base at the back. 

Very carefully level the instrument by turning the levelling 
screws so that the pointed pin, attached to the moving magnet, 
is just over the cross marked on the fixed stud under it. The 
moving system should now be quite free, at all events laterally. 
See therefore that this is the case. 

The height of the moving magnet can be adjusted to give 
freedom of motion by carefully turning the milled-headed pin 
which passes into the torsion head at the top of the instrument. 
Set the torsion head with its pointer to zero, and then bring the 
magnet pointer to its zero by turning the wooden base which 
carries the coils. Fix the instrument in this position by turning 
the milled-headed pin which projects from under the base. In 
using the instrument thus adjusted to measure current, place it 
directly across the low resistance provided, and to measure 
higher voltages connect it in series with the separate anti- 
inductive resistance provided with it, when the extremities of the 
combination will then be the terminals of the voltmeter. 

Caution. —Make quite sure that the correct resistance is 
plugged in, otherwise the instrument may be fused up. The 
plug may be used as a make and break key. Being a + and — 
instrument, it must be connected up in circuit that the magnet 
pointer tends to move in the opposite direction to that of the 
torsion head 


Siemens Electro-Dynamometer. 

This instrument depends on the electro-dynamical action of one 
circuit which carries a current on another circuit carrying either 
the same or a different current, and is illustrated in Fig. 251. It 
consists of a base supported on three levelling screws and carrying 

P P 


578 


ELEGTRIGAL ENGINEERING TESTING 


an upright standard, to wliicdi latter is fixed two distinct stationary 
coils usually wound with two different gauges of wire and number 
of turns so as to obtain a wider range of sensitiveness and 
measurement than would be possible with only one fixed coil. 

A movable coil, the plane of which is perpendicular to that of 
the fixed coils in the normal position of the former when actually 



Fig. 251; 


measuring a current, is suspentled by means of a silk fibre from a 
torsion head, at the top of the instrument, carried at the centre 
of a graduated sale, which is itself screwed to the top of the 
standard. 

A rather long helical spring, composed of a sufficient number of 
turns, has one end fixed to the under side of the torsion head and 






ELEGTBIGAL ENGINEERING TESTING 


579 


the other to the top of the moving coil, which is thus controlled 
by the turning of the head. Electrical connection is made with 
the moving coil through two mercury cups into which its ends 
dip and which are directly in a line, under the point of suspension. 
A milled-headed pin or rod, carried by a light support, seen at the 
back of the scale, passes through the hollow torsion head, and has 
attached to it the silk thread that suspends the movable coil and 
which passes down through the torsion head. Hence, by turning 
the rod round the swing coil can be raised or lowered so as to 
clear the other fixed fittings. The moving coil, which may consist 
of more than one turn, can be raised and clamped during transport 
by a spring clamp (not seen in Fig. 251) at the back, controlled 
by a milled nut. The instrument requires to be carefully levelled 
before using to ensure perfect freedom with the moving coil. The 
levelling screws and spirit-level are added for this purpose, though 
sometimes a plumb line is used in place of the latter. The fixed 
and moving coils are in simple series; one end of each of the fixed 
coils goes to the outside terminals, the other ends both to the top 
mercury cup and the lower cup to the centre or common terminal. 
Thus there are two sensibilities, viz. the moving coil in series with 
either fixed one, according to whether the centre and left or centre 
and right pair of terminals are in use. Since at the actual 
moment of measuring a current by the dynamometer, the fixed 
and movable coils are always in the same position {i. e. their axes 
perpendicular) relatively to one another, due to the index pointer 
on the moving coil always being brought back to zero by turning 
the torsion head, the couple or force, whether of attraction or 
repulsion, exerted by one coil on the other is oc C/j x x G-^ 

oc G-^ where Gi and G^ are the currents in the two coils which are 
equal or the same. But this force is just balanced by the force 
of torsion exerted by the spring az angle of torsion or the deflection 
D of the torsion head. Hence, D cc (7^, 

. •. (7 oc Df 
or (7 = 717^77 amperes, 

where K is the constant for the particular fixed coil used which 
gives an equation of equality. 

This is the law of the Siemens electro-dynamometer. 

Some of these instruments have scales divided into numbers 
oc to the square roots of the usual divisions, and in such cases the 


580 


ELEGTRIGAL ENGINEERING TESTING 


current G = Kx scale reading simply. In using these instruments 
care must be taken to either twist the “ leading in and out ” leads 
together, or run them very close so that the swing coil may not be 
affected by the current in these leads. 

In calibrating or using the instrument with direct currents, it 
must be so placed that the plane of the suspended coil when in 
its zero position is perpendicular to the plane of the magnetic 
meridian of the earth. The reason for this is, that when the swing 
coil carries a direct current it is acted on by the earth’s magnetism 
independently of the action due to the current in the fixed coil, 
and the position of rest for the first cause is when the planes of 
the magnetic meridian and swing coil are at right angles. For 
alternating currents there is no such action. 


Siemens Dynamometer-Wattmeter. 

Except for the swing or movable coil, this instrument is 
precisely similar to the preceding dynamometer. It is illustrated 
in Fig. 252, which indicates two or three alterations to the general 
form of the Wattmeter, which the author has thought it bene¬ 
ficial to make. The one illustrated is provided with two thick 
fixed coils, as in the dynamometer, Fig. 251, connected directly to 
the three large terminals in the middle, so that two distinct 
sensibilities can be obtained instead of usjually only one. 

The swing coil now consists of many turns of fine insulated 
wire wound on a light rectangular frame of ebonite or boxwood. 
Only a few of the turns are wound inductively, the rest being 
doubly wound and therefore non-inductive. The total resistance 
of the swing coil is, however, that due to the sum of all the turns, 
which may amount to 5000 ohms or more. Current is led into 
and out of the swing coil through thicker wires soldered to the 
fine wire and which dip into the two mercury cups. These last- 
named are directly connected to a separate pair of small terminals 
seen on the extreme right and left, having no electrical connection 
whatever with the thick coils. 

The scale is provided with a mirror for the purpose of avoiding 
errors due to parallax in reading the position of the torsion head 
pointer. The mirror glass covers the scale, but a circular strip of 


ELECTRICAL ENGINEERING TESTING 


581 


silvering is removed just over the scale, enabling it to be seen but 
preventing it getting dusty and dirty. In all other respects this 
Wattmeter is the same as the dynamometer shown in Fig. 251. 
When the swing coil is returned to its zero, by turning the torsion 
head, we have as before its deflection Z) x x C' 2 > but if = the 



Fig. 252. 


main current and C 2 tbe current in the fine coil, which is placed 
across the mains, and therefore is x to the voltage ( F), we have 
BaoC.Vcc Watts, 

or Watts (IF) = KI), 

where K is the constant of the instrument for the thick coil used. 
Thus by combining the voltmeter and ammeter in one and the 




582 


ELECTRICAL ENGINEERING TESTING 


same instrument, the deflection of the new instrument so formed 
measures the power in Watts absorbed by any circuit. Though 
the Wattmeter is of no great use in direct current work, since we 
usually require both the amperes and volts separately and can 
always multiply them and so obtain the power when desired, the 
instrument is of incalculable value in alternate current work, since, 
if nearly non-inductive, it is the best known means of obtaining 
the true 'power in such a circuit, the product = amps, x volts not 
giving this quantity. 

The same precautions are necessary in using the Wattmeter as 
in the dynamometer, and in addition errors may arise through the 
warming up of the swing coil and consequent alteration of its 
resistance, as in the case of electro-magnetic voltmeters. The 
error that may be introduced by the earth’s field is explained 
on p. 580. 


Change-over Switch. 

Fig. 253 represents a form of switch suitable for largo currents 
and which can be used in one or other of four ways as follows— 

(1) As a single-way, single-pole switch by connecting to the 
centre and either of the end terminals on the same side.- 

(2) As a single-way, double-pole switch by connecting the centre 
and an end terminal on the same side in one main, while the other 
centre and corresponding terminal at the same end is put in the 
other main. 

(3) As a two-way double-pole switch by connecting the common 
circuit to the two centres and each branch circuit to the pair of 
terminals at one end. 

(4) As a reversing switch by connecting the circuit or portion 
to be reversed to the centre pair and the main current to the 
pair at either end, cross connecting the corner terminals at the 
ends by temporary wires. 

The figure shows the construction fairly clearly, and it consists 
of two metal blades carried by smaller extensions at their lower 
extremities and capable of turning about a horizontal axis in the 
upright standards which form part of the base carrying the middle 
terminals and inner wedge blocks. Four similar wedge blocks 


583 


ELEGTRIGAL ENGINEERING TESTING 

(two at each end) are carried by metal bases on which the four 
end terminals are fixed. When the blade levers are up (as seen), 
the metal parts, electrically connected to the six terminals, are 
insulated from each other, as the lever blades are also insulated 
from each other, their upper extremities being fixed by an insulator 
cross-piece to which the handle is fixed. When the blades are 



Fig. 253. 


pushed down one side or the other into their respective pairs of 
wedge contact blocks they short circuit these, thus joining the 
centre and end terminals on one side together, and likewise those 
on the other side. 


Keys. 

A form of key, which, though not very portable, is extremely 
useful in a test room, is shown in Fig. 254, and is otherwise known 
as a Pohl’s commutator. It consists of a wooden, though prefer¬ 
ably polished ebonite, base Jf, containing 6 small pure-copper 









584 


ELEGTBIGAL ENGINEERING TESTING 


mercury cups ABCDEF^ each suitably connected to a terminal 
(not shown). 

A polished, and preferably corrugated, ebonite rod or bar G is 
supported on two pure-copper pillars the lower ends of 

winch dip into and are securely pinned to the cups B and E 
respectively, so as to be capable of rocking backwards and forwards 
in B and E. G carries two curved pure-copper rods rg, connected 
electrically to and S 2 by the copper strips p^ respectively. 
Thus as G is rocked to one side or the other, so and are 
dipping simultaneously into cups G and F respectively, or A and 
D, Fig. 254 showing the latter position. Consequently in this posi¬ 
tion A and B will be in electrical connection and also D and E with 



Fig. 254. 


G^ turned over then B and G are connected instead and also E 
and F. 

It will now be evident that such a rocking key can be used in 
one of four ways— 

(1) As a single-way, single-pole key by connecting to B and A 
or B and C, and likewise at the other end. 

(2) As a single-way, double-pole key by connecting one wire to 
B and .4 and the other of the pair to E and D. 

(3) As a 2-way double-pole key by connecting the common 
circuit to B and E and the branches to AD and CF 
respectively. 

(4) As a reversing key by connecting the circuit to be reversed 
to B and E and the main circuit to either A and D ov G and F^ 
cross connecting A and F and also G and D. 

This kind of key or Pohl’s commutator is sometimes convenient 









ELECTRTGAL ENGINEERING TESTING 


585 


to use in insulation resistance work, and for such, the base M 
should be of well-polished ebonite, quite clean and free from dust, 
while the cups and terminals should not be too close together. 
It is an advantage also for G to be corrugated and well polished, 
so that leakage to the hand on turning G over is reduced to a 
minimum. Owing to the key having mercury cups, it cannot 
be said to be portable in the ordinary sense of the word 

A less elaborate though equally useful key is shown in Fig. 255. 

If anything it is a little more port¬ 
able than the Pohl’s key, but cannot 
be operated quite so quickly. It will 
perform the same four offices as the 
above key, but has the advantage that 
no cross connection is necessary in 
order to be able to employ it as a 
reversing key. The principle of con¬ 
struction is shown in Fig. 255, which 
represents a plan of the key. On a 
wooden or preferably a polished ebonite 
base B are four terminals T-^ 
connected by pure-copper strips (shown black) to mercury cups 
a, 5, c, and d, 

A cubical block of polished ebonite P capable of sliding up and 
down on, and also of turning on, a central metal pin, carries two 
pure-copper strips A 2 , the ends of which terminate in legs 

which dip into the cups a dj. 

In the position shown, terminals T-^ and are in electrical con¬ 
nection through and also and through /Ig. If, however, 
P is raised out of the cups, turned round 90°, and again slipped 
into the cups, then and will now be in connection through 
Ag (if P was turned clockwise), and through Ky The 

key can obviously be used in the following ways 

(1) As a single way, single pole key by joining to any pair of 

adjoining terminals, e. y. T-^ and ^g. 

(2) As a single-way, double-pole key by connecting the pair of 
incoming leads to any adjacent pair of terminals such as 

and the out-going to the opposite pair 1\ and Ty 

(3) As a reversing key by joining the main leads to either 
pair of diagonally opposite terminals, e. g. and and 





58r) ELECTRICAL ENGINEERING TESTING 

the circuit to be reverse I to the other pair ^2 ^4 

vice versd. 

(4) As a 2-way key by joining one main to any one terminal, 
e. y, I\ and the branches to 2\ and T^. 


A Highly-Insulated 2-Way Spring Tapping 

Key 

is shown in Fig. 256, and consists of a well-polished ebonite base 
B, supporting at one end a well-polished ebonite standard S, to 
which is fixed two brass contact terminal blocks 

Let into and carried by the top of the standard S is an 
ebonite rod A, which at its other end supports an ebonite 



block E. To E is fixed two springy brass strip levers L-^ L^, 
electrically connected together and to the common terminal T at 
that end. 

The free ends of the spring strips are provided with rather long 
ebonite knobs for the finger of the operator to tap. 

In the use of this key, if the high-potential wire is connected to 
the common terminal the path of any leakage lies from T 
across E, then along R and down S to the base B, and thence to 
earth. This being long gives the key a high-insulation resistance. 














ELEGTIilGAL ENGINEERING TESTING 


587 


and to still further increase this, all the ebonite parts should bo 
well polished and quite clean and free from dust. 

It will be noticed, that since there is a lever to each way, it 
would be possible to press both at once. Unless otherwise directed 
this must be absolutely avoided, as serious damage may be done in 
consequence. 

Fig. 257 shows, in plan, a convenient form of 2-weij sliding switch. 
It consists of a wooden or ebonite base B, to which is fixed three 
terminals T, and T\. The two latter, and T^, make permanent 
contact with the contact blocks Jg respectively, while T acts also 

as a centre for the contact lever L 
to turn on. The knob K is merely 
for the purpose of conveniently turn¬ 
ing the lever L. 

In the position shown in the 
Figure, there is no connection be¬ 
tween and either 2\ or but by 

turning L so as to rest on or 

then connection is made between T 
and or between T and respect¬ 
ively. 

It will be noticed that only one 
contact can be made at one and the same time. This is an 
advantage in some cases, where thf^ simultaneous making of 
both ways might cause an accident. 



Arc Lamp Photometer Cradle. 

In the photometrical testing of electric arc lamps it is 
necessary to be able to have the means of measunng the candle- 
power of the arc in several directions, making various angles with 
the horizontal line passing through it. In some cases this is done 
by raising or lowering the lamp vertically, the beam from it 
being reflected along the bench by a fixed plane mirror suitably 

placed, but capable of rotating on an axis. ^ ^ ^ 

The author has devised the cradle shown in Fig. 258, m which 
the lamp to be tested is placed. The lower part is a rigid frame- 







588 


Fig. 258. 


ELEGTBIGAL ENGINEEEIN<} TESTING 


Fig. 259 







































ELECTRICAL ENGINEERING TESTING 


589 


work which is placed on the photometer bench and carries an 
upper frame capable of moving round a hollow tubular horizontal 
axis, which latter is itself capable of moving in the fixed frame and 
has attached to it a pointer, seen 
in Fig. 258, just above the two 
large terminals. In this upper 
frame is placed the arc lamp, 
which can therefore be inclined 
at any angle to the vertical, as 
read off by the pointer on its 
scale. 

The carbons of the Lamp are 
first adjusted so as to touch at 
a point in the axis of rotation, 
as seen through the tubular axis. 

Thus the arc in turning as the cradle is turned, ailways main¬ 
tains the same position relatively to the photometer bench, and 
the author has found that next to no difficulty in the regulation 
of the arc by its auto-mechanism occurs up to 50° or 60° from 
the vertical. After this the carbons have to be partly regulated 
by hand. 



Direct-Reading Bar Photometer. 

This form, due to Mr. Trotter, has a direct-reading scale on its 
bank which shows without calculation the ratio between the 
standard and lamp under test when the sight-box seen in the 
middle of the bank (Fig. 259) is moved so as to obtain an equal 
illumination of the screens. The general arrangement is very 
suitable for making rapid tests on electric glow lamps, as slight 
variations of E.M.F. affect both lamps equally and do not cause 
appreciable errors. Fig. 260 shows a sectional plan of the sight- 
box ER, in which A A are the apertures at the sides to admit the 
beams of light from the two lamps AjAg to be compared. SS are 
two screens, the illuminations of which are compared. One of 
these contains a star-shaped hole for the purpose of enabling the 
further screen to be seen through the funnel or window A, through 
which the observer looks. 











590 


ELEGTRIGAL ENGINEERING TESTING 


Illumination Photometer. 

Tliis is a portable direct-reading instrument devised by Mr. 
A. P. Trotter, by which the illumination at any spot in a street 

or building can be at once measured in terms 
of the illumination given by an amyl-acetate 
standard of light, this being found more 
reliable and less troublesome than ordinary 
standards of light. A general view of the 
instrument is shown in Fig. 261 1 and a sec¬ 
tional elevati«n in Fig. 262. 


Instructions for Use. 

Remove the end cap with the mirror 31 
on it by the bayonet joint; remove the 
cover from the lamp and light it, then re¬ 
place the end cap. The flame can now be 
seen in the mirror. The top of the flame 
should just touch the point of the bent arm. 
The flame is raised or lowered by screwing the lampholder in or 
out by its lower end. By unscrewing the holder completely it 
can be drawn out for refilling. 

Having adjusted the flame to the right height the cap on the 
top is removed and the photometer is set so that the paper screen 
S is horizontal. 

The general illumination to be measured falls on this screen S. 
Across the centre of it is a small slit T through which an inside 
screen R illuminated by the standard lamp can be seen. The 
observer’s eye must be vertically over the hole in the screen. 
The inside screen R is then adjusted by the outside arm F. 
When the illumination of the outside screen S is greater than 
that of the inside, the slit T will appear dark; when less the slit 
will appear bright. With a little practice the slit can be made 
to nearly, if not quite, vanish; the illumination is then shown 
on the scale F by the pointer P. The unit used is the illumina¬ 
tion due to one standard candle at one foot; that is, if a balance 
is obtained when the pointer is at “ 1,’^ the illumination is equal 



Fig. 261. 










ELEGTRIGAL ENGINEERING TESTING 


591 


to that of 1 standard candle at 1 ft. distance, if at “ 2 ” the 
illumination is twice this, and if at “ *1,” one-tenth of that which 
would be given by a standard candle a foot away. 

The inside screen R has a slight blue tint which to a great 
extent removes the colour difficulty. 

A slightly yellow upper screen is provided for measuring with 
very blue lights. 

Be careful the eye is vertically over the slit. Holding one 



anger near the eye and between it and the liole will make it 
easier to get the vertical line. 

Be careful to stand so that the body is not between the screen 

and any source of light. 

The slit must be square across the instrument. 

See that the height of the flame is right before and after each 
reading; the flame sometimes increases a little, especially just 

after being lighted. 













































































592 


ELECTRICAL ENGINEERING TESTING 


The flame is not afliected by a gentle breeze, and on windy 
nights the instrument can easily be shielded by a piece of card¬ 
board or brown paper. 

Only pure amyl-acetate must be used, and the wick must be 
cut square across without ragged points. 

Keep the cap on over the screen when not in use, and do not 
let the screen get dirty. 

There is a small pin at the centre of the pivot of the arm; 
when the instrument stands so that its shadow thrown by any 
lamp falls on the scale, it gives the cosine of the angle of 
incidence, having which the actual C.P. can be calculated. The 
height of the lamp is equal to the distance from the post when 
the shadow falls at 45“. 

Photometer Screens. 

Of these there are many different types, all of course effecting 
the same purpose, namely, that of enabling equality of illumina¬ 
tion due to two different sources of light to be visibly determined, 
and hence their relative intensities. Fig. 263 shows a form of 
Bunsen screen arranged inside a “ sight-box ” seen with its top or 
lid open to enable the inside to be seen. It consists of two discs 
of plain paper having its centre portion greased in the form of 
a star. These discs are seen one near each end of the “ sight- 
box,” which is dull black inside and prevents stray light due to 
external or internal reflection from getting to the Bunsen star 
discs or screens. Two vertical plane mirrors are placed as shown, 
symmetrically with regard to the discs, and each at 45“ to the 
back of the box. 

The images of the two discs can be seen in the two mirrors 
through two rectangular open windows in the box shown in front 
of Fig. 263, and thus the intensities of their illuminations can be 
compared by the eye. It should be noticed that the sight-box 
illustrated in Fig. 263 cannot give true results, since the ratio of 
the squares of the distances from the light sources to the star 
discs an'd again to the centre of the box are not the same. The 
box is given, however, to indicate this fact and to act as a 
warning to users of it. Another form of this screen is shown 



ELEGTIilCAL ENGINEERING TESTING 


593 


resting on the top of the “ sight-box,” illustrated in Fig. 264. 
Here there is but one disc with a plane mirror on either side of 
it, making equal angles with it, and enabling an image of each 
side of the screen to be seen without looking directly at the disc 
itself. 

The lower part of Fig. 264 illustrates a form of balancing 
screen due to Jolly, and consisting of two rectangular blocks of 
clear paraffin wax, cemented together but separated by a film of 
silver paper so as to prevent direct transmission of light right 



Fig. 263. 


through the two blocks. They are placed inside a sight-box to 
prevent stray light getting to them, hence the two blocks will 
appear equally bright when the two sources of light to be com¬ 
pared, placed on either side of the sight-box, illuminate them 

equally. 

One of the best photometer screens is that due to Messrs. 
Lummer and Brodhun, and is shown symbolically in Fig. 265. In 
this type the light from the two sources to be compared falls on a 
screen SS^ having equally light surfaces. The rays are then 

Q Q 


















594 


ELECTRICAL ENGINEERING TESTING 



Fia. 265 











ELEGTlilGAL ENGINEERING TESTING 595 

reflected by two similar plane mirrors and to two prisms 
Pi and 1 2, of which P^ is the ordinary form, while P, has a 
curved surface one side which touches P^. 

The reflected rays from wij are able to pass through the com¬ 
bination to the eye placed at E, while those from are deviated 

to E also. Thus the screen SS when equally illuminated both 
sides cannot be seen at E. 


“ Methven Screen ” Photometric Standard 

of Light. 

When used with just an ordinary amount of care this standard 
of light is one of the most convenient and accurate, requiring 
no elaborate preparation, as in the case of some other standards, 
before being used. A standard two-slot “ Methven screen,” together 
with a carburetter, is shown in Fig. 266. The former consists of 
a vertical brass plate or screen, bent round at right angles at the 
bottom, and to the under side of which is fixed a tubular metal 
foot which fits into the hollow standard supporting the whole 
screen. 

To the upper side of this angle-piece is fixed a London Argand 
burner provided with a stettite cone and a cylindrical glass 
funnel. Two pairs of brass bars are screwed into the screen at 
respectively and 3" above the top of the burner. The screen 
has a hole at its centre across which slides a silver plate 
containing two rectangular windows, the size of which are 
determined by the height of the flame and the C.P. of the light 
emitted from them. This in the Methven screen, shown in 
Fig. 266, is 2 C.P. either when the mean height of the peaks of 
the flame are on a level with the two top bars and the long 
narrow slot in use with ordinary coal-gas, or with the flame on a 
level with the two lower bars, the short broad slot and the coal- 
gas carburetted. The carburettor is shown on the left of 
Fig. 266, and is merely a metal reservoir containing pentane 
liquid, which is highly volatile, the vapour mixing freely with 
the coal-gas and enriching it as this latter is made to pass 
through the receptacle by manipulating the three stop taps seen 


596 


ELECT RIGAL ENGINEERING TESTING 


on the branch tubes. For a more detailed description of the 
Methven screen, see Slingo and Brooker’s Electrical Engineering. 



Fig. 266. 


It should be noticed that the position of the screen is rather 
misleading, arising from the fact that it is shown turned round 
out of its normal position to show more clearly the burner, etc. 


Adjustable Carbon Rheostat. 

An extremely useful form of continuously adjustable rheostat, 
suitable for large currents as well as small ones, is illustrated in 
Fig. 267, this particular one being capable of carr 3 dng about 
25 amperes continuously or 30 to 40 amperes for short periods 
of some minutes’ duration. 

It consists of a row of square flat plates of hard gas-retort 
carbon, resting on a fairly broad ledge of slate or some other 
suitable insulating material screwed to an iron bar underneath 



















:ELEGTEIGAL engineering testing 507 

it which is fixed to the ends of the framework of the rheostat. 
These ends are also fixed to tie rods at the sides, to the top of 
each of which is screwed an over-lapping strip of vulcanized fibre 
to guide the plates, and at the same time not to short-circuit 
them. The row of carbon plates are terminated by thick cast- 
iron plates, extended at the top and carrying the terminals 
shown. The left-hand terminal plate is insulated from that end 
by a plate of vulcanized fibre, while the row of plates is com- 



Fig. 267. 

pressed against this by a screw working against the right-hand 
end. In case at any time the right-hand terminal plate is 
removed and inserted in some intermediate position in the row, 
an extra cast-iron plate the same size as the carbons is provided 
this end to guard against the friction of the carbons should the 
screw press on them by mistake. One valuable advantage of this 
rheostat is that, being non-inductive, it can be used with alter¬ 
nating currents in cases where many other forms of rheostats 
could not be. The resistance varies from a minimum when the 
plates are tightly compressed, to a maximum when they are loose. 








598 


ELEGTEIGAL ENGINEERING TESTING 



Incandescent Lamp-Box Resistance. 

In many tests on alternating current appliances, such as trans¬ 
formers, alternators, etc., difficulties arise in obtaining a non- 
inductive resistance in which to take up the output of the appli¬ 
ance, for, as is well known, the product of the volts and amperes 
only represents the actual or true 'power in Watts absorbed, 'pro¬ 
viding the load-absorbing rheostats are truly non-inductive. 


Fig. 268. 


This can be obtained with specially wound rheostats made of 
ordinary iron wire or other special alloy. A water rheostat is, 
however, simpler, though not perhaps so convenient to manipulate, 
while Fig. 268 illustrates a still more convenient form of non- 
inductive (to all practical purposes) rheostat, which the author 
has designed for his own purposes, and one that is fairly portable 
with care. The particular one shown consists of a containing 
case and box, with an internal partition which supports some 60 


rv;gr 




















ELECT RIGAL ENGINEERING TESTING 


699 


or 70 glow lamps, composed of 8, 16 and 32 C.P. lamps, capable 
of absorbing some 7 or 8 E.H.P. All the woodwork the lamp 
side of the partition is covered with a double layer of asbestos 
cloth, and the front of the box is protected only by a grating to 
allow of free circulation of cold and hot air. Each lamp is con¬ 
nected to its own pair of mercury cups in a special switch-board, 
seen on the top of the box towards the back, by means of which 
the lamps can at once be connected all in parallel, all in series, or 



Fig. 269. 

in any other intermediate combination, to suit requirements. 
The side opposite the grating hinges down, giving free access to 
the lamp-holders and connections to the mercury switch-board. 
This lamp-box resistance requires care in carrying about, as the 
mercury tends to come out of the two long slots if the box is 
much tilted. It is, however, extremely convenient, and when 
the lamps are used in parallel in alternating current work, their 
effective self-induction is extremely small, so that when absorb¬ 
ing the load from the secondary side of a transformer (say), the 
amps. X volts wffl be the true power absorbed, e. developed by 

the transformer. 














600 


ELECTRICAL ENGINEERING TESTING 


Adjustable Rheostat. 

Fig. 269 shows a convenient wire-wound rheostat, capable of 
step-by-step variations between 0 and the maximum resistance, 
which is about 40 ohms, and of carrying continuously 6 or 7 
amperes. It is a form of rheostat eminently suitable for 
regulating the shunt circuit of either a dynamo or motor of that 
type. As will be seen with reference to the figure, it consists 
of a box or case containing the coils (not seen), and which is 
fitted with a top carrying the multiple way lever switch of the 
form shown. The circular row of studs seen in the upper part 
of Fig. 269 are connected to-the coils inside in such a way 
that the right-hand end block gives the full resistance of all the 
coils in series between the terminals at the top, when the lever 
is on this block. The rheostat, being intended to be used in a 
vertical position, has a wire grating at the top and bottom in 
order to obtain a cooling circulation of cold air through the in¬ 
terior. The coils inside are strung between insulators, so that 
even if they do get very hot, the inside of the box will not be 
much harmed. 


Continuously Variable Rheostat. 

This rheostat is of the same type and make as that illustrated 
and described relative to Fig. 271, consequently a further descrip¬ 
tion is unnecessary. By winding the rheostat with, say, the same 
gauge in a higher specific resistance material than platinoid, such 
as eureka, manganin, or reostene, it is easy to obtain a resistance 
which can be varied perfectly continuously from something like 
30 or 40 ohms down to 0, and that will carry a maximum 
current of about 4 or 5 amperes. This in a test-room or 
laboratory is extremely useful, enabling very fine adjustments of 
resistance, and consequently of current or pressure, to be obtained 
when desired. 

The reader is referred to the description of the same make of 
rheostat shown in Fig. 271. 



ELEOTEICAL ENGINEERING TESTING 


601 


Improved Rheostat. 

The object of the rheostat, invented over forty years ago by 

Wheatstone, is to provide an electric resistance which can be 
varied continuously. 



Fig. 270. 


The instrument shown in Fig. 271 is an improved form (due 
to Lord Kelvin) of Wheatstone’s rheostat, in which the wire is 
guided from one cylinder to the other by a fork carried along 
through the requisite range by a nut travelling on a long screw- 











602 


ELECTRICAL ENGINEERING TESTING 


shaft. This screw-shaft carries a toothed wheel which turns the 
two cylinders by means of toothed wheels attached to th.eir shafts. 
A watch-spring, as in John’s improvement of Wheatstone’s 
rheostat, keeps the wire always tightened to the proper degree. 
A leather buffer at eacli end of the range of the nut acts as a 
guard against overwinding in either direction. 

In a high resistance rheostat the conducting cylinder and 
the wire are both of platinoid, a metallic alloy having properties 
which make it specially suitable for the purpose. It has very high 
electric resistance, very small temperature variation of resistance, 



Fio. 271. 


and its surface remains almost or altogether untarnished in the 
air. On account of the last-named property the contact between 
the wire and the conducting cylinder, and continuity in action, 
which was a great difficulty in the old form of apparatus, is very 
complete. 

In a low resistance rheostat the conducting cylinder in this 
instrument is made of brass, nickel-plated so as to avoid tarnish¬ 
ing, and the wire used is copper, also nickel-plated. The rheostat 
can be supplied to carry currents as high as 30 amperes. The 
relation between material resistance and current for these 
rheostats is as follows, viz.— 


























603 


BLEOTBIGAL ENOINEEBINO TESTING 



Table XIV. 


Wire. 

Approximate 

Maximum 


Resistance. 

Current. 

Platinoid. 

600 ohms. 

0*25 amperes. 

»> 

100 „ 

2-0 „ 


20 „ 

10-0 

Copper. 

0-4 

30-0 „ 


Continuously Variable Rheostat. 

Fig. 2/2 illustrates another slightly different form of Kelvin's 
improvement on the original Wheatstone rheostat. The actual 



Fig. 272. 


construction is the same as that described in the two preceding, 
Figs. 270 and 271, except that the one here shown is intended 
for finer and lighter work, as seen from the smallness of the 
parts and gauge of wire employed. The second cylinder is just 
behind the one shown in the figure. Fairly fine wire is used, and 

















ELECTRICAL ENGINEERING TESTING 


cm 

the rheostat so wound may have a resistance of about 100 ohms 
as a maximum, capable of carrying one or two amperes. Like 
the “ Wirt ” form of resistance, described in Practical Elec¬ 
trical Testing by the author, it forms a most useful type of 
resistance for small work, such as calibrating low resistance 
voltmeters, enabling a series of different readings to be taken 
without actually altering the number of cells in the E.M.F. 
used. 

The reader should refer to the description of the principle of 
this form of rheostat which is given relative to Fig. 271. 


Fixed Standard Low Resistances. 

There are many different forms of these depending to a great 
extent on the value of the resistance, and also on the particular 
make. Fig. ’273 shows a set of five different forms of standard 
low resistances made by Messrs. Crompton and Co., and 
primarily intended for use with the potentiometers made by 
them also. The resistances can, of course, be used for any other 
purpose than this which requires a standard resistance of accur¬ 
ately known value, capable of carrying large currents without 
sensible heating or alteration. 

The one shown standing on end at the top is of a slightly 
different form to the rest, being of the tubular water-cooled 
type. 

These resistances consist of a sheet or strip of metal, or a coil 
of wire, each provided with four terminals, two for connection to 
the circuit and two for connection to the potential leads. 

The smaller resistances take the form of a coil or spiral fixed 
in a mahogany frame, and also of flat strips either bent or 
straight, the largest size—300 amperes and over are of sheet or 
water-cooled type. 

They are constructed of manganin, an alloy which has been 
thoroughly tested, and which has the great advantage that within 
ordinary limits of accuracy (say one part in 1000) no tempera¬ 
ture correction whatever is necessary: but for measurements 
with the potentiometer requiring an accuracy exceeding this, a 
curve, giving the temperature value for the whole range of 


w 



ELECTRICAL ENGINEERING TESTING 


605 



current that the instrument is capable of carrying, is supplied 
with each resistance. 


Fig. 273. 


Such a curve, together with other forms of adjustable and 
fixed standards of low resistance, are given in the author’s 
work entitled Fractical Electrical Testing. 


m 

















































































































































































































































ELEOTBIGAL ENGINEERING TESTING 


()0C 


Stand Coil Rheostat. 

A most convenient form of current rheostat of a portable 
nature, at all events one that can be moved comfortably about 
any testing-room, is illustrated in Fig. 274. It consists of an 



Fig. 274 . 


iron frame or stand, of as light a construction as possible, in 
order to be light, between the top and bottom of which are 
stretched bare wire spirals of either iron or some high-resistance 
alloy. These coils are spaced sufficiently far apart to prevent 
them easily touching should the rheostat receive a slight knock, 
and are all connected in series, their junctions being connected 



















ELECTRICAL ENGINEERING TESTING <)<)7 

to a multiple way switch seen on the top. This latter consists of 
several studs or blocks arranged in a circular form and having 
their upper surfaces turned up in the lathe so as to be quite 
level. A suitable spring lever pivoted in the centre of the ring 
of blocks is capable of turning almost once round between 
two stops and of making contact with each block as it passes 
over it. 

One of the two terminals of this stand coil rheostat (seen on 
the top) is connected to the lever centre, and the other to one end 
of the series of coils. Thus the resistance between the terminals 
can be varied from nothing (^. e. short circuit) to the maximum 
by as many steps as there are contact blocks on the switcli. A 
great mistake, which is usually made by rheostat makers, is to use 
the same gauge of wire throughout the rheostat, for clearly the 
gauge should increase as we begin to cut out, since the current is 
thereby increased also. The above rheostat, made to the author’s 
designs, contains about four or five gauges of wire. 


Three-Phase Liquid Rheostat. 

For three-phase alternating current work there are two dis¬ 
tinctive forms of rheostats needed, differing merely in the 
terminal arrangements. One form may be termed a “ throizgh ” 
rheostat, which would be required for regulating the current 
supplied by a generator to, say, a motor; the other form may be 
termed a “ closed ” rheostat, which would be required for absorb¬ 
ing the load from a generator. 

In all forms the rheostat must operate equally on each of the 
leads of a three-phase system, otherwise the balance and sym¬ 
metry of the currents will be thrown out and will cause con¬ 
siderable trouble. Three-phase rheostats may be either metallic 
or liquid in nature, but in any case the moving contacts must 
move simultaneously by equal amounts when manipulating the 
rheostat as a whole. Fig. 275 shows a three-phase liquid rheostat 
designed by the author, and which will negotiate currents up to 
about 30—40 amps. It can be employed either as a “through” 
or “ closed ” rheostat, and consists of three exactly similar flat 
semicircular-shaped iron troughs or boxes placed side by side in 


608 


ELEGTRIGAL ENGINEERING TESTING 


alignment on a wooden base board, but not in contact with one 
another. Each is secured to the base board through its flat 
metal foot, carrying a terminal which therefore makes electrical 
connection with the box as a whole. 

These terminals are clearly seen in Fig. 275. Each box has a 
semicircular-shaped iron plate, carried by a brass spindle, which 
passes through bearings of insulating material let into the 
opposite sides of each box. The ends of each spindle terminate 
just outside the box in enlarged metal bosses, the successive 
adjoining pairs being direct coupled mechanically through coup¬ 
lings of insulating material, but are discontinuous electrically. 



Fig. 275. 


This compound shaft or spindle is rotated with the plates by 
means of a worm and worm-wheel gearing. Seen to the right- 
hand end of Fig. 275. 

Three other terminals (not seen in the figure) at the back of 
the rheostat make electrical connection to each section of the 
spindle, and therefore to each plate through spring strips rubbing 
on the proper bosses as the spindle is turned. 

A solution of washing soda and water of the same density is 
used in each trough, and must not, of course, reach up so high 
as to make contact with the spindles. Thus when none of the 
terminals are cross-connected, the rheostat becomes a “ throitgh 
type,” but when the three at one side are all joined together, we 








ELECTRWAL ENGINEERING TESTING 


im 

have a closed rheostat, suitable for absorbing the load from a 
generator connected to the remaining three terminals. 

It should be remembered that as each of the three sections of 
this water rheostat must operate equally on turning the shaft, 
the level of liquid must be the same in each trough in addition 
to it being of the same density. 


Magnetic Curve Tracer. 

This instrument, devised by Prof. Ewing, shows the magnetic 
quality of iron, steel, or other magnetic metal, by exhibiting the 
cui've which connects the magnetization B with the magnetizing 



Fio. 276. 


force II in any magnetizing process. . The curve is exhibited 
upon a screen by the spot of light reflected from a mirror, which 
receives two components of motion. The vertical component 
is proportional to the magnetization, and the horizontal com¬ 
ponent to the magnetizing force. The instrument is sliown 
in Fig. 27G, and Fig. 277 is a diagram showing the func¬ 
tions of the various parts. The mirror is .pivoted upon a single 
needle-point, which leaves it free to turn both ways, and it is 
connected by threads to the middle of two stretched wires, 
A A and BB, in such a manner that when either of the wires 
sags the mirror suffers a corresponding deflection. The threads 













ELECTRICAL ENGINEERING TESTING 


(UO 


1 


are kept taut by light springs, the tension of which is adjustable. 
The wires are stretched in narrow slots, forming gaps in two 
magnetic circuits, DD and G, One of these circuits, EDy is 
made up of the iron or steel to be examined, along with suitable 
pole-pieces and yoke, and the current which passes through the 
magnetizing coils of this circuit passes also through the stretched 
wire, RR, in the gap of the other magnet. The other magnet 
is constantly magnetized by a steady current, and a steady 
current also flows • through the stretched wire AA. Hence, 



when the magnetizing current of DD is altered, the wire RR 
sags out or in, and gives horizontal motion to the mirror pro¬ 
portional to the magnetizing force acting on DD. And when 
the magnetism of DD is altered, the wire AA sags up or down, 
giving vertical motions to the mirror proportional to the changes 
of magnetism. 

The samples to be tested form the arms of DD. They may 
be solid rods, or rods built up of thin strips, or of wire. The 
rods supplied with the instrument are of soft sheet-iron, built 
up of insulated strips, with a net cross-section about 1 in. by 
\ in., and about 18 in. long. In preparing other rods for 
comparison of magnetic quality, the same dimensions are to 






















ELEOTlilGAL ENGINEERING TESTING 


ini 

be chosen as those of the standard rods. Clamps are provided 
at the pole-pieces to allow the rods to be readily inserted and 
removed. 

The same constant current will serve for the wire AA, and 
the magnetizing coil of the tubular magnet C. A current of 
about four amperes will serve well, but more or less may be used, 
according to the amount of movement which it is desired to give 
to the mirror. The amplitude of the movements can also be 
regulated by shifting in or out the weights on the bell-crank 
levers which keep the stretched wires tight. For high-speed 
work the wires should be kept very tight, and a small mirror 
should be used. The magnetizing current must be made to vary 
in a continuous manner; not by sudden makes and breaks. 
When these precautions are taken magnetic cycles may be per¬ 
formed so rapidly that the reflected light appears on the screen 
as a continuous curve. A special commutator is supplied, to allow 
of rapid but gradual variations and reversal of the magnetizing 
current. It consists, essentially, of two fixed and two revolving 
plates of zinc, immersed in a solution of zinc-sulphate. 

In ordinary testing it is more convenient to make the magnetic 
changes occur slowly, and to mark with a pencil the successive 
positions of the spot on the screen. A sheet of paper on a small 
drawing board, set up against a wall or other vertical support, 
is a suitable screen. The source of light may be an ordinary gal¬ 
vanometer lamp, furnished with a pair of cross wires instead of 
the usual single wire. For high-speed work a small spot of light 
is necessary, which is obtained by placing a screen with a small 
hole in it just in front of the lamp. Horizontal and vertical 
datum lines are marked by moving (by hand) the wires BB and 
AA respectively, and marking the path of the spot. A variable 
resistance is to be inserted in the magnetizing circuit of I)D, to 
allow successive points of the magnetizing curve to be obtained : 
two zinc plates suspended near together in a weak solution of 
zinc-sulphate, in such a way that they can be more or less deeply 
immersed, will serve well for this purpose. A rapid commutator 
is also to be put in this circuit, to allow the specimens under 
test to be demagnetized by rapid reversals of continuously 
diminishing magnetizing force, if it is wished to determine 
the curve of initial magnetization. In comparing other samples 


612 


ELEGTRIGAL ENGINEERING TESTING 


with the standard bars, care must be taken to preserve the 
same scale of R and of II, by not changing the constant current 
in the wire AA and magnet G, nor the tension of the stretched 
wires. 

The absolute scale of II may be calculated, if required, from 
a knowledge of the number of turns in the winding of the 
magnet limbs, and that of B may be found by means of an 


P 



Fig. 278. 


auxiliary ballistic galvanometer, by winding an induction coil 
of a few turns round one or both of the limbs. 

Fig. 278 shows the electrical connections with the terminals 
as they are placed on the slate base of the instrument. 

Terminals 1 and 2 are those of the coil which magnetize the 
tubular magnet C; terminals 5 and 6 are those of the main 
magnet coils on DD. The constant current is supplied by P, 





























ELEOTRIGAL ENGINEERING TESTING 


613 


.and taking the course 1, 2, 3, 4, passes in series through the 
magnetizing coil ot C and the stretched wire A. The variable 
current is supplied by Q, through the commutator K and 
adjustable resistance R. Taking the course 5, 6, 7, 8, it passes 
through the m.ain m.agnetizing coil and the stretched wire B. 
The copper strip which is used to connect 2 with 3 may be 
put between 1 and 3 instead, 2 and 4 being then the battery 
terminals in that circuit. The effect of this is to change 
the general slope of the figure on the screen from right to 
left or vice versd. One of the two arrangements is right 
when an ordinary screen is used; the other is right when the 
screen is a piece of tracing paper or ground glass, viewed from 
behind. 

Care should be taken to adjust the instrument so that when 
a complete cycle of magnetic reversal is performed, the figure on 
the screen will be symmetrical, with the extremities equidistant 
from the zero point or origin, which corresponds to the condition 
of no current in B and no magnetism in DD. To secure this, 
see that the stretched wires are as nearly as may be judged in 
the middle of their respective slots, both vertically and horizont¬ 
ally. Set the mean position of the mirror perpendicular to the 
pivot needle, by adjusting the needle’s position by the screws 
provided for that purpose. The light springs which keep the 
connecting threads taut are to be set so that they remain con¬ 
siderably stretched even when the mirror moves to its furthest 
limit in the direction tending to sl.acken them. 

Fig. 279 is an example of the curves obtained by the instru¬ 
ment shown in Fig. 270. It is the copy of one half of a cyclic 
curve of reversal, along with the initial curve taken after 
demagnetizing the specimen by reversals, and also the curve 
obtained by reapplying the magnetizing current after it had 
been reduced from its maximum to zero. Owing to the existence 
of an air gap in the magnetic circuit under test, the diagram 
is sheared over to the right, and true values of the magnetic 
force would be found by me.asuring horizontal distances from 
some such line as YY, instead of from the vertical line. This 
shearing does not affect the area enclosed by the cyclic curves 
of reversal, and need not therefore be taken account of in 
measuring the comparative amounts of energy dissipated by 


614 


ELEGTRWAL ENGINEERING TESTING 


magnetic reversals in different specimens or in the same 
specimens with different values of the magnetism. 

In addition to its use for determining these areas, for com¬ 
paring the magnetic qualities of different samples of iron, and 
for investigating the properties of magnetic curves generally, 
the instrument may be used as a galvanometer by making the 

6 



current to be measured pass through either of the stretched 
wires, while the magnet, in the slot of which the wire is 
stretched, is kept in a constant state of magnetization. This 
will be found useful in cases where an extremely dead-beat 
indication is wanted, and by making the spot of light register 
its position photographically on a moving plate or paper satis¬ 
factory records of rapidly fluctuating currents may be obtained. 






ELEGTRICAL ENGINEERING TESTING 


615 


The Permeameter. 

A useful piece of apparatus, known as the “ Permeameter,” is 

illustrated in Fig. 280, by means of 
which the magnetic quality of dif¬ 
ferent materials can easily and 
rapidly be found. The method is 
essentially a workshop one, and the 
principle of it is due to Professor 
S. P. Thompson. 

• The arrangement consists of a 
somewhat massive hollow rectangu¬ 
lar-shaped block of good soft 
wrought-iron forged to the shape 
shown. Inside this is a magnetiz¬ 
ing solenoid wound on a thin brass 
tube with thin flanges or ends, its 
length being just that between the 
insides of the block ends. The 
sufiiciently long rod of magnetic 
material to be tested, having its 
lower end faced quite true, passes 
freely but closely through the top 
end of the block, down througli 
the solenoid, and beds on the care¬ 
fully “faced” inside of the bottom 
end of the block. The protruding 
end of the rod has a metal pin 
through it, which is caught by a double hook on the lower end 
of an ordinary spring balance, the top end of which is suspended 
by a gut cord passing over a fixed pulley and attached to the 
lever shown. This permeameter is also fitted with an arrange¬ 
ment for testing the specimen ballistically. It consists of a 
small flat coil fixed to a brass plate, which slides backwards and 
forwards between guides. The rod passes through this coil and 
beds as before on the block, at the same time keeping the coil 
back against the force of two spring strips on the outside of the 



Fm. 280. 








H16 


ELECTRICAL ENGINEERING TESTING 


block. Immediately the rod is suddenly pulled up the coil flies 
out, and a circuit joined to its terminals will receive an electro¬ 
magnetic impulse proportional to the field just broken. In this 
way the ballistic and traction methods can be made to check 
one another in the final results obtained. 


Condensers. 

Fig. 281 shows a general view of the Kelvin standard air 
Leyden condenser, and Figs. 282 and 283 a plan and sectional 



Fig. 281. 


elevation of the same. The instrument is formed by two mutually 
insulated metallic pieces, which we shall call A and B, constituting 
the two systems of the air condenser or Leyden. The systems 
are composed of parallel plates, each set bound together by four 
long metal bolts. The two extreme plates of set A are circles of 
much thicker metal than the rest, which are all squares of thin 







ELECTRICAL ENGINEERING TESTING 


617 


sheet brass. The set B are all squares, the bottom one of which is 
of much thicker metal than the others, and the plates of this system 
are one less in number than the plates of system A. The four 
bolts binding together the plates of each system pass through 
well-fitted holes in the corners of the squares; and the distance 
from plate to plate of the same set is regulated by annular 
distance pieces, which are carefully made to fit the bolt, and are 
made exactly the same in all respects. Each system is bound 



firmly together by screwing home nuts on the ends of the 
bolts, and thus the parallelism and rigidity of the entire set is 
secured. 

The two systems are made up together, so that every plate of 
B is between two plates of A, and every plate of A, except the 
two end ones, which only present one face to those of the opposite 
set, is between two plates of B. When the instrument is set up 
for use, the system B rests by means of the well-known “ hole, 




















618 


ELEOTIUGAL ENGINEERING TESTING 


slot, and plane arrangement,” ^ engraved on the under side of its 
bottom plate, on three glass columns, which are attached to three 
metal screws working through the sole-plate of system A. These 
screws can be raised or lowered at pleasure, and by means of a 
gauge the plates of system B can be adjusted to exactly midway 
between, and parallel to, the plates of system A. The complete 
Leyden stands upon three vulcanite feet attached to the lower 
side of the sole-plate of system A, 



Fia. 283. 


In order that the instrument may not be injured in carriage, 
an arrangement, described as follows, is provided, by which 
system B can be lifted from off the three glass columns and 
firmly clamped to the top and bottom plates of system A. 

The bolts fixing the corners of the plates of system B are made 
long enough to pass through wide conical holes cut in the top and 

^ Thomson and Tait’s Natural Philosophy, § 198, example 3. 















































































































































ELECTRICAL ENGINEERING TESTING 


619 


bottom plates of system A, and the nuts at the top end of the 
bolts are also conical in form, while conical nuts are also fixed to 
their lower ends below the base-plate of system A. Thumbscrew 
nuts,/, are placed upon the upper ends of the bolts after they 
pass through the holes in the top plate of system A. 

When the instrument is set up ready for use, these thumb¬ 
screws are turned up against fixed stops, so as to be well clear 
of the top plate of system A ; but when the instrument is packed 
for carriage, they are screwed down against the plate until the 
conical nuts mentioned above are drawn up into the conical 
holes in the top and bottom plates of system A ; system R 
is thus raised off the glass pillars, and the two systems are 
securely locked together so as to prevent damage to the in¬ 
strument. 

A dust-tight cylindrical metal case, k, which can be easily 
taken off for inspection, covers the two systems, and fits on to a 
flange on system A. The whole instrument rests on three vul¬ 
canite legs attached to the base-plate on system A ; and two 
terminals are provided, one, i, on the base of system A, and the 
other, J, on the end of one of the corner bolts of system R. 

Revolving Contact Maker. 

This is an arrangement of two contact levers or spring strips, 
and a revolving ring, whereby electrical contact is made between 
the two strips once every revolution of the ring at a certain 
particular and definite instant, and place on the circle of revolu¬ 
tion depending on the position in which the levers touch the ring. 
Such an arrangement is necessary when it is desired to take the 
periodic E.M.F. and current curves of an alternator, and it may 
be fitted either to the alternator, or to a small single-phase 
synchronous alternating current motor, to be driven off the 
particular supply to be sampled. Fig. 284 represents a simple 
and convenient form of revolving contact maker, designed by the 
author and fitted to the rotating inductors of an inductor alter¬ 
nator. It consists of a brass frame ring screwed to the 
alternator portion, and carrying an ebonite ring screwed to it 
securely by set screws through the inner edge of the frame ring. 
Half the total width of this ebonite ring is turned nearly away 
and a brass ring slipped on and fixed to the ebonite ring by 


620 


ELECTRIGAL ENGINEERING TESTING 


screws passing through it sideways. A thin slip of brass (seen 
just in front of the spring brushes in Fig. 284) is neatly let into 
the ebonite ring and makes electrical contact with the insulated 
brass ring only. Two spring strips, insulated from one another, 
and pressing against the two springs in a line, are carried by a 
holder capable of being slid along the curve bar seen in the figure 



Fia. 284, 

and provided with a pointer moving over a scale. Hence any 
circuit connected to these strips or brushes will be closed once 
every revolution at an instant in the period which depends on the 
position of them on the curved rod. 

If the toes of the spring brushes are set, one in front of the 
other, the contact will be more instantaneous, otherwise it will 
last during the whole time required for the slip contact to pass 
under the brushes when set level. 





ELECTRIGAL ENGINEERING TESTING 


f)21 


Cradle Absorption Dynamometer. 

In Fig. 285 is shown the principle of a typical form of cradle 
absorption dynamometer, suitable for test ng the horse-power 
developed by small motors, but which can also be employed for 
larger powers up to a certain limit determined by the weight and 



size of the motor, and the consequent difficulty in constructing 
the dynamometer. 

It consists of a light framework {CC') carrying a small floor or 
platform at its lower extremities to which the motor M, to be 
tested, is bolted after being packed up so that the centre of its 
shaft is a line with the points of bearing 0 of the knife edges K. 
The steel planes on which K work are at the top of standards F 
carried on a light bed-plate FP» A block (6) is attached to the 






















































022 


ELECTRICAL ENGINEERING TESTING 


top of the frame cc, which carries a screwed bolt on which can 
travel a heavy balance weight B. By raising this weight, there¬ 
fore, the centre of gravity of M, its bed-plate and the cradle CC 
can be raised to the level of 0. A light lever LI is fixed to and 
moves with CCy one end being attached to the piston (y>) of 
a dash-pot D containing some viscous fluid for damping the 
motions of the cradle cc. 

The other end is attached to the lower portion of a suitable 
spring balance TT, itself supported from a bracket carried by a 
standard {xx). 

The end of the lever L moves in front of an index scale ss, also 
carried by xx. 

The tail rod together with the piston y), are made to, once for 
all, balance the other part of the beam L, so that after B is 
properly adjusted the cradle with its motor would rest in equili¬ 
brium in any position if was disconnected from L. 

The method of procedure is therefore as follows—When the 
ci'adle is exactly balanced in the manner indicated above, the 
spring balance W is attached to Z and the nut N screwed up or 
down so as to bring the lever L to the zero of the index scale SS. 
The motor M is now connected up so that the field magnets and 
consequently the cradle tend to turn counter-clockwise. This 
condition must be arranged for beforehand, and the motor placed 
in the cradle accordingly, to suit the orthodox direction of 
rotation of the armature A. 

A cord is now wrapped once round the motor pulley, and its 
two ends stretched out horizontally. The motor will thus be 
made to do work against the friction between the cord and 
pulley, and the result will be a depression of the end L of the 
beam. Then bring L back to zero on the index scale by turning 
N and so raising the balance JV. 

If now W = reading of the spring balance in lbs. (say), and L 
= the distance in feet of its point of attachment to the lever, 
from the centre 0, then the moment of the force resisting 
rotation, i. e. the torque T= WL (pound feet), and if the speed 
of the armature = 9^ revs, per sec., the angular velocity a>=. 27 r 9 i. 

Hence the work done per sec. = ooZ, and the horse power 

developed =—^, since 1 H.P. = 550 ft. lbs. per sec. 

550 



ELEGTRIGAL ENGINEERING TESTING 


(523 


Horse-Power Transmission Dynamometer. 

When the mechanical power required to drive some particular 
machine, as for instance a dynamo, is desired to be known, it can 
be obtained by means of a transmission dynamometer. This is 
an appliance for measuring mechanical power without absorbing 
any of it, as distinguished from the absor'ption dynamometer 
which measures the power by wasting it all. There are two main 
classes of the transmission instrument, namely, those for measur¬ 
ing the power transmitted directly through a shaft, and secondly, 
those for measuring the power transmitted by a belt. 



Fig. 286. 


Fig. 286 represents the general view, and Fig. 287 the sym¬ 
bolical side elevation, of one belonging to the latter class and 
made by Messrs. Siemens Bros, and Co. of London. By means 
of it the difference in tension {T — I) between the driving (i. e. 
the tight) and slack sides of the belt can be read off directly in 
pounds, and which is the only one troublesome factor required 

of the horse-power to be measured. 

Referring to Fig. 287, this Siemens transmission dynamometer 
consists of four similar roller pulleys, running in bearings carried 
at the four corners of a light but strong iron framework (^KK), 
Three other roller pulleys run in bearings carried by the arm or 














































f524 


ELEGTRICAL ENGINEERING TESTING 


frame (A), which is capable of oscillating about a fulcrum F on. 
part of the main frame. The centre pulley P is really the 
actuating part of the dynamometer, the remaining six, namely p 
and g, merely acting in a sense as guide pulleys for the belt, the 
“tight”*or driving side of which is TT and the slack side tt. 
The left-hand end of the arm or frame A has attached to it a 
link (1), which actuates a lever pointer L capable of moving on a 
fulcrum (/) over an index scale and carrying a balance weight 
W for the purpose of balancing the arm A with its fittings, etc. 
Z) is a dash-pot to steady, what would otherwise be, the jerky 




Fig. 287. 


movements of A. A strong, spiral spring capable of being 
extended by turning a handle (/^), is attached to the left-hand 
part of A. The action of the dynamometer will now be fairly 
obvious, and is as follows— 

The tight side of the belt TT tends to force down the left-hand 
end of A against the smaller upward force of the slack side tt, 
this causes L to turn about/in a counter-clockwise direction ; but 
L is now brought back to zero by turning (4), and when this is 
the case, the force exerted by S just balances and is equal to the 
difference in tension {T — t), and is read off on the scale over 







































ELEGTEICAL ENGINEERING TESTING 


025 


which the pointer, attached to the top end of S, moves. Thus 
according as to whether S is calibrated in lbs. or kilograms so 
the quantity {T — t) is read off in these units. If now V = 
velocity of the belt in ft. per min., as obtained from the noted 
speed of the driven pulley {N) revs, per min. and its diameter 

d (ft.), then V=7rdN, and the H.P. transmitted = 

^ ^ 33,000. 


The Measurement of Power transmitted 

by Belts. 

In the case where it is desired to measure the mechanical power 
absorbed by some particular machine, such as, for instance, a 
dynamo feeding a lamp circuit, or any other kind of machine 
driven by means of belting, the use of a form of Prony brake or 
other kind of absorption dynamometer is inadmissible, owing to 
the well-known fact that such an appliance wastes all the power 
which it measures. In tliis case a transmission dynamometer has 
to be resorted to, of which there are sevenal different forms, some 
remaining permanently in position, so as to be capable of indicat¬ 
ing at any moment the power transmitted, others being tempor¬ 
arily erected in position for the tests—as, for instance, the 
Siemens-Hefner-Alteneck belt transmission dynamometer. It is, 
however, manifestly more convenient to have a permanent 
arrangement, and the general principle of this type is to connect 
the driving pulley, which is loose on its shaft, by three or more 
helical springs, to a fixed collar or boss keyed to the shaft, 
and then to measure in some convenient way the “angular 
advance” of the shaft relatively to the pulley, due to the axial 
extension of the springs. This, as is well known, gives a 
measure of the power transmitted. Hitherto, however, the 
arrangements for observing this angular advance have not 
proved very satisfactory. 

By the kind permission of the proprietors of The Mechanical 
Engineer, the author is enabled to give a reprint here of an article 
written by him in that journal of March 19, 1898, on a neat form 
of spring transmission dynamometer the recording arrangement of 



626 


ELECTRICAL ENGINEERING TESTING 


which was devised by Professor W. Stroud some little time 
back, and is in use in the electrical engineering laboratories 
at the University, Leeds. It is accurate and extremely simple 
both in principle ami wor-king, and can be easily fitted to any 
shaft and kept permanently in position. It has the distinct 
advantage that the measurement of angular advance is solely an 
electrical one, and can consequently be obtained with considerable 
accuracy. 

Fig. 288 shows an end elevation, and Fig. 289 a side eleva¬ 
tion, of the arrangement, with the lower part of the driving 
pulley (y) cut away. It is on a counter-shaft, and rotates in the 
direction of the arrow (Fig. 288), driving a machine. It consists 
of a boss 0, which is keyed to the shaft, and is provided with a 
flange Q, having an extension at one part of its circumference in 
the form of a short projection or arm. One end of the steel¬ 
driving spring M, which is of square cross-section, is bent so as to 
partly embrace this arm and be driven by it; the other end is 
similarly bent, only in the opposite sense, to partly embrace one 
arm of y, which is loose on the shaft, and drive it. Only these 
two bent ends of M are shown in Fig. 289, two turns being 
cut away, as shown, in which is a pai t sectional elevation about 
a vertical diameter through shaft centre. The belt can be 
thrown on to a loose pulley {u) by a fork not shown. To the 
short projection on the flange Q (Fig. 289) is bolted a light but 
strong bent arm A into a saw-cut, in the end of which are sweated 
the ends of two rigid strips of brass side by side. The right-hand 
one ends in a hinge, which carries a similar strip J, into the side 
of which is fixed a rigid pin, which passes freely through a slot 
cut in the end of the left-hand fixed strip. This latter is merely 
for the purpose of preventing J being pulled too far towards the 
pulley (y) by the spiral spring V. The hinged strip J carries at 
its end a light brass block IF, to which is attached, but electric¬ 
ally insulated from it by means of ebonite or vulcanized fibre (/>), 
a rounded brass contact block This makes electrical contact 
with a curved resistance frame RS^ consisting of a curved piece of 
wood of somewhat smaller radius than the pulley rim, and of 
section something similar to that shown at R (Fig. 289). It 
embraces an angle of about 120 deg., and is provided with shallow 
saw-cuts on the inner and outer peripherios. Into these are 


ELEGTEIGAL ENGINEERING TESTING 


627 






\ 


X 


Fig. 288. ^^9 





















































































028 


ELEGTBIGAL ENGINEERING TESTING 


pressed the turns of wire with which it is wound, consisting in 
the present instance of between 280 and 300 turns of No. 18 or 
No. 20 B. W. G. platinoid wire, double silk covered, the ends 
being led out to brass terminal blocks at the extremities E and S. 
The arrangement is securely clasped to the arms of the pulley by 
the counter-sunk bolts N^ which pass through a similarly curved 
piece of wood of section shown at X (Fig. 289), placed on the 
other side of the arms. A thin piece of soft insulating material 
{B) is interposed between these latter and E to prevent some of 
the turns of wire being short-circuited, and thereby rendered 
useless, or damaged by pressure against the arms. The turns- of 
wire are bared of their silk insulation, along the line on which T 
makes contact with them between E and S. Two thin strips of 



Fig. 290. 

vulcanite 11 are bent completely round the shaft, and over them 
are stretched two thin strips of brass CG, the ends of each being 
soldered together so as to form continuous rings, against which, 
as they rotate, press two small copper gauze brushes It 

now remains to describe the method of indicating the angular 
advance, and so the measure of power transmitted. This is 
accomplished wholly electrically, a symbolical diagram of con¬ 
nections being shown in Figs. 290 and 291, corresponding parts 
being lettered alike. The brass rings CG are permanently and 
electrically connected by insulated copper wires to S and 1\ as 
shown, the shaft and arm A being depicted as taking up a position 
P on ES in advance of the pulley (F). E is in electric connection 
with F, and therefore with the shaft. E' is a fixed brush, which 






























ELECTRICAL ENGINEERING TESTING 


r>29 


simply rubs on and makes contact with the shaft. K is a two-way 
key; E a battery of about three cells, preferably secondaries; G is 
a sensitive galvanometer or potential difference indicator, as dead 
beat as possible, and preferably of the moving coil or D’Arsonval 
type, in order that, firstly, its deflections may be directly pro¬ 
portional to the potential difference at its terminals ; and, secondly, 
that by winding the moving coil on a light but broad aluminium 
frame, it can be made very dead beat. G is provided with a 
“ constant total current shunt,” shown symbolically at H (Fig. 
290), which is for the purpose of adjusting the sensibility of G, to 
which it is shunted without altering the gross resistance of the 
combination, and therefore the P.D. between the two points to 
which it is applied. 



The principle of action of II is merely that the screw, flxed as 
regards end play, when turned, actuates the contact block, rubbing 
between the top and bottom resistances, which are suitably pro¬ 
portioned to one another and to the galvanometer as well. The 
resistance between the terminals of the combination should be at 
least 20 times that of IIS. 

In starting the calibration, switch the lever of key (K) on to 
stop 1 (Figs. 290 and 291). G is then directly across A, and the 
number of cells together with the sensitiveness of G (by using II) 
should be adjirsted to give a full scale deflection. This must 
always be re-obtained before starting subsequent tests. Now, 
switch to 2 on K, so completing the main circuit by way of F, 
shaft, 7?, S, A, E, and back to F. A certain current will flow, 
















FLECTIUGAL ENGINEERING TESTING 


()30 

depending, of course, on the total resistance and pressure at 
and for this current a definite P.D. will exist between R and 
and also between R and P, causing a deflection on G, as shown. 
The calibration is now finally effected by hanging known dead 
weights from the face of pulley (F), thereby twisting up M (Fig. 
289), and noting the corresponding deflections on G. The mean 
nett pull with any particular dead weight will best be obtained by 
taking the mean of two readings on 6r, corresponding to the two 
extreme positions of equilibrium of the pulley, this latter being 
helped to take up these positions. 

Fig. 292 shows two calibration curves A and B, for two totally 
distinct transmission dynamometers of this type, every detail in 
each being precisely the same with the exception of the springs 
M, being different as regards number of spirals and area of cross- 
section of these only. A few details about these may be worthy 
of note. The spring of the dynamometer giving curve A (Fig. 292) 
consists of three complete turns of tempered steel of normal in¬ 
ternal diameter =• 6square cross-section = in. x -^1- in. 
A pull = weight of 50 lbs. at the pulley face wound it up tight on 
the surface of the bosses 5-| in. diameter, giving a deflection on G 
of 9T (10 being full). This result is clearly shown by the b<^nd- 
ing of the curve at the top part. The spring of the dynamometer 
giving curve B consists of five complete turns of same diameter, 
but square cross sections = in. x ^ in. Its internal diameter w'^as 
approximately ^in. in excess of that of its bosses in.), for the 
largest weight used (180 lbs.). Hence, not being tight up, there is 
no bending of the curve B. The turns of both springs nearly 
touch. 

It may now be useful to note, with regard to the design of 
such a helical spring, the relation that subsists between the pull 
of the weight at the pulley face, the decrease of the diameter of 
the spiral, and the angular deflection of one end of the spring in 
a plane perpendicular to its axis, represented by the position or 
the point of contact P (Figs. 290 and 291) on RS. Let TF=: 
dead weight applied, i.e. the tangential force at the circumference 
of the pulley of radius R. 

Then the moment {M) of the twisting couple in a plane per¬ 
pendicular to the axis of the spring is M— WR. If the coils are 
quite flat, and their planes at right angles to this axis, there will 


ELECTRICAL ENGINEERING TESTING 


031 


be no torsion in the spring itself, and it will be wholly subjected 
to a bending action due to M. 

If o'q = mean radius of the helix before W is applied, 
r = mean radius of the helix after W is applied, 
and ^ = angle of twist, {. e. the angle through which it is wound 
up. Ihen, since the moment of the twisting couple must just 
balance that due to the elastic force of the spring when the arm 
A (Fig 290) has come to some steady position F on RS, we have 



HI = El = 7 ~ WR^ where length of the spring 

and El represents the flexural rigidity of the material of which 
the spring is made. 

E being the modulus of elasticity for the material, i. e. tempered 
steel. 

I being the moment of inertia of the section of the material, i. e. 
for a square. 

V 

In ihe present case, therefore, for square section / = 






































































































632 


ELECTRICAL ENGINEERING TESTING 


(b) =r length of a side of that square section. Substituting in the 
above equation we have 

12 \r 


WR = 


0 ' 


12-1 


from which it can be seen that for a given pulley, material of 

. (sectional area of spring)^ 

the spring, and angle of twist, It oc - length - 

We have also that the deflection or distance through which W 
acts = R6. 

Writing the first equation in another form, we get an expression 
for the amount of coiling of the spring for a particular weight W. 

Thus 


Tq WR + El 


Utq WR + Eb^ 


W is evidently the nett pull on tlie circumference of pulley, and 
if :Z’= tension (in lbs.) of the tight or driving side of the belt, and 
^ = tension (in lbs.) of the slack side ; then W — (T — i) = nett pull 
of belt in pounds, which is one factor of the horse-power trans¬ 
mitted. Also, if the pulley of the machine driven by (Y) is of 
radius {/) ft. and makes (n) revolutions per minute, then, for 
no slipping of the belt, velocity of belt (v) = (2Tr/n) ft. per 
minute, and horse-power 

transmitted = 

33,000 33,000 ^ 

If the driven machine be a dynamo developing A amperes of 
current at a pressure of V volts on the external circuit at that 

A V 

speed, then the useful horse-power developed II^ = 


Its commercial or nett efiicieiicy is therefore = 100 per cent. 

In conclusion, it may be mentioned that the dynamometer 
forms a simple and accurate means of measuring the horse-power 
transmitted to any machine, and works very satisfactorily pro¬ 
vided there is no skidding of the belts, and also that the spring 
(F) (Fig. 289) exerts sufticient tension on J to give a good 
reliable contact between the contact piece (T), to which the 
brass ring under is electrically attached, and the curved 
lesistance RS. 










ELECTRICAL ENGINEERING TESTING 


633 


Absorption Dynamometer. 

Fig. 293 illustrates an absorption dynamometer which is a 
modification of a Prony brake for making brake tests of the 
horse-power developed by electromotors. One great trouble in- 



Fig. 29-1 


herent in all such tests is the high speed (relatively to other 
prime movers) at which these machines run, especially small ones. 
This tends to cause a jerkiness of the brake and trouble conse 


/ 

















634 


ELECTRICAL ENGINEERING TESTING 


quently in reading the indications of the various parts. In Fig. 
293, three plies of cord make a half-lap over the top side of the 
brake pulley, at one side supporting a scale pan, and at the other 
(the nearest to the observer looking at the figure) being attached to 
a single cord passing round a nearly frictionless pulley and hang¬ 
ing from tlie lower end of the spring-balance seen at the top of 
the illustration. This double bend of the cord is merely for con¬ 
venience in having the balance in a position where it can be read 
easily. The three plies of cord over the pulley are kept in posi¬ 
tion by light brake blocks as shown. Thus the weights in the 
pan will represent the tension on the tight side, and the reading 
of the balance that on the slack side, so that their difference 
gives the nett load on the brake. The brake pulley shown is a 
special form of box pulley devised by the author, and water-cooled 
by an inlet or outlet pipe passing through a central opening. 

Another form of Prony brake, giving excellent results with 
small motors from about ^ B.H P. upwards at speeds up to 



2,000 revs, per min., is shown diagrammatically in Fig. 294, and 
consists of a light cast-iron flanged pulley P keyed to the shaft 
S of the motor to be tested and rotating truly on it. A light 




















ELECTRICAL ENGINEERING TESTING 


635 


fiamework FFF carries an eyelet E, from which hangs a spring 
balance w. h rom the lower enci of a pliable cord passes 11 
times lound P and is attached to another spring balance W. From 
the upper end of JV a cord (7^ passes round a small grooved 
pulley A (supported from F) to a fixed point R on an iron lever L f. 
This lever is pivoted at / and is capable of moving in a guide G 
between the limits L and Xh If P rotates counter-clockwise as 
shown, (IF) will read the tension on the tight side, and [w) that 
on the slack side, of the brake rope (7, so that {W — w) = nett 
pull in lbs., and if (r) = radius of (pulley face -}- J diam. of O) 
in feet, then the torque exerted = (JV — w) r Ib.-ft., and the 

T-) TT "p 27rnr(TF — i <» • 

= -g g QQQ -, where n — speed of P in revs, per min. 


The ranges of IF and w may be as 3:1, and if the face of P 
rotates quite truly^ the deflections of w and TF are quite steady 
and easy to read. The hand pressure on L can bo both easily 
and very gradually applied, and when released, the weight of IF 
raises L and releases P from the pressure of CG automatically— 
a feature of some value in preserving the rope, and a convenience 
in motors which do not race on removal of load. 


Eddy Current Brake. 

One of the most convenient methods of measuring the brake 
liorse-power of electro-motors, petrol, and other motors, especially 
when the speed is high, is by means of an electro-magnetic or 
eddy-current brake. While the principle has been used in 
commercial work to an enormous extent in light apparatus, 
e. y. in electricity meters and instruments, so far as the author is 
aware, Messrs. Morris & Lister of Charlton Works, Coventry, 
were the first to patent and put on the market a form suitable 
for absorbing and measuring the B.H.P. of electro-motors, etc. 
One of these, used by the author for many years, gave excellent 
results. Unfortunately the firm have ceased to make them, and 
it is therefore of little use to describe this particular form. The 
general principle underlying the construction and action of all 
such brakes will readily be understood from Fig. 295. 






636 


ELECTRICAL ENGINEERING TESTING 


A cylindrical ring C of copper is fixed securely to the outer 
surface of an iron drum 1 keyed to the shaft {S) of the motor, 
and rotates in a multipolar electro-magnetic field system F. 

Fixed to and in a line passing through the centre of is a 
light graduated lever Z, from two to five feet from tip to shaft 
centre, and made of thin aluminium or steel tube. This moves 
opposite a fixed index scale B between stops A, and carries a 
light slider 1) from which known weights W can be hung. A 
small sliding weight w, which can be clamped by a set screw on 
a light tail rod (^), serves to counterbalance the weight of L and 
D. A direct current, flowing through the field coils of A, pro 
duces a powerful multipolar magnetic field through the gap 
between I and the poles in which C rotates. E.M.F.’s are 
therefore induced in C, causing eddy or Foucault curi'ent>- 



to flow in (7, which oppose the driving source, and the output of 
the motor is thus expended in heating C. The strength of these 
currents, and hence the B.H.P. of the motor absorbed, depends 
on the strength of field, which is readily controlled by means of 
a rheostat in the exciting circuit. 

The force or torque resulting from the interaction between 
field and currents in C tends to drag the floating-field system 
F round, and this is opposed and balanced by a gravitational 
force due to the weighted lever L. 

When a position of D (at a radius R feet from the shaft 
centre) in conjunction with the adjustment of excitation, is 
found, such that the above opposing forces balance, the lever L 
will float between the stops A. The torque or turning moment 
(in lb.-ft.) exerted by the motor then = WR and its B.H.P. = 









ELECTRICAL ENGINEERING TESTING 


637 


27rRnW 

33,000 


at a speed of (n) 


revs, per min. 


The field frame F can either be carried by a separate support, 
giving freedom of oscillation, or on a sleeve by the motor spindle 
itself; but even in this method the weight due to the whole 
brake is considerably less than the pull due to a bell drive. For 
example, a 3-H,P. (continuous rating) size, made by Messrs. 
Morris & Lister, and running at 1,500 revs, per min., weighed 
45 lbs., while a 35-H.P. (continuous rating) brake at 250 revs, 
per min. weighed 250 lbs. complete. With a suitable provision 
in the design for ventilation, such brakes could be used for 
temperature load tests, but these are more economically effected 
by one or other of the well-known regenerative methods. A 
brake suitable for a certain H.P. and speed (intermittent rating) 
will, of course, absorb smaller powers for longer periods, or one 
for continuous rating larger powers, up to 100%, for short 
periods. Higher speeds require smaller brakes. Anyone who 
has used this class of brake will have realized the many practical 
advantages it possesses over other kinds of absorption brakes, 
particularly in the matter of sensitiveness, control, smoothness 
of operation, and accuracy in repeating readings, to say nothing 
about the absence of wear, burnt blocks, bands, and rope and 
water-cooling arrangements. 


Soames Motor Testing Brake. 

This appliance belongs to the class of mechanical power 
measurers known as absorption dynamometers, and is a modified 
form of Prony brake made by Messrs. Nalder Bros, and Co. It is of 
extremely simple construction, and gives perfectly definite weigh¬ 
ing of the torque on the pulley under test against ordinary dead 
weights. It consists of a steel lever working on knife edges, 
which can be raised or lowered by the hand-wheel at the top of 
the brake. Holes are drilled in the lever at equal distances from 
the centre, corresponding to the ordinary sizes of pulley in use. 

The centre of the brake is placed over the centre of the pulley, 
and from the two holes corresponding to the diameter of the 
pulley under test is suspended a piece of webbing, which passes 
round the pulley as in the diagram, Fig. 296. 



038 


ELECTRICAL ENGINEERING TESTING 


When the pulley is running, a weight, say from ^ to 30 lbs., is 
suspended on the end of the arm as shown. 

The whole is then raised by turning the hand-wheel, tightening 
the belt on the pulley until sufficient friction is put on the belt 
to raise the arm to a horizontal position and keep it floating 
there; the speed of the pulley being taken at the same time. 

The weight is hung at either of the two holes at the end of the 
bar marked respectively H.P. K— jjjqq and Watts A"o= the one 



hole giving H.P. direct by multiplying weight in lbs. by revolu- 
tions per minute, and dividing by 4000, the other hole similarly 
giving Watts direct by dividing by 6. 














ELECTRICAL ENGINEERING TESTING 


639 


The constant for H.P. is 27r//-^33,000, L being the distance of 
the weight from the centre. 

The size of the pulley does not enter into the equation, so long 
as the distance between the holes in the arm is equal to its 
diameter. 

The band must, in all cases, be hung from two holes equidistant 
from the centre. 

The pulley should be perfectly smooth and flat, and it is con¬ 
venient but not necessary to have it flanged. 

No lubricant is generally required, but a little black-lead may 
be applied if necessary ; no oil. 

This brake is extremely accurate, and every reading can be 
repeated with certainty, each one not taking more than ten 
seconds at the outside. 


t 


TABLES OF 


CONSTANTS, LOGARITHMS, ETC. 

Standards for Copper Conductors. 

{Adopted hy the Electrical Standards Committee representing the 
General Post Office, Institution of Electrical Engineers, and all 
the leading cable manufacturers of Great Britain.) Revised 
March 1910. 

A wire 1 metre long, weighing 1 gramme, at 60° F. (15*6° C.) 
has Matthiessen’s value of resistance— 

0-1539 standard ohms for hard-drawn, high conductivity 
commercial copper. 

0-150822 standard ohms for annealed, high conductivity com¬ 
mercial copper. 

These figures at 60° F. being calculated from 0T469 per metre- 
gramme for hard-drawn, and 0T440 for annealed copper at 32° F. 
by Matthiessen’s formula— 

D _ ^^32° 

1 - 0-00215006 {t - 32) -f 0-00000278(« - 32)2 

Hard-drawn copper is defined as that which will not elongate 
more than 1% without fracture. 

Copper is taken as weighing 555 lbs. per cubic foot at 60° F., 
and its corresponding specific gravity = 8-912. 

The average temperature coefficient is = 0 00238 per degree F. 

A lay of 20 times the pitch diameter is taken as the standard 
for calculating all tables. 

The resistance and weight of conductors is calculated from the 
actual length of wire, or 1-01226 times the length of the cable for 
all except the centre wire. 


640 




ELEGTRIGAL ENGINEERING TESTING 641 

Maximum variation of resistance or weight of any wire allowing 
for losses in manufacture is 2^. 

An allowance of 1% increased resistance as calculated from the 
diameter is permissible on all tinned copper between Nos. 22 and 
1 2 gauges inclusive. 

The following tables of figures are deduced from the above 
constants— 


Table XV. 


Copper Weighing 655 lbs. per Cubic Foot at 60° F. 


Solid Wires. 

Resistance in Standard Ohms of high conductivity 
Commercial Coi)per. 

Annealed. 

Hard-Drawn. 

Resistance per cubic inch 
») j> >> cm. 

,, of 100 ins. 

weighing 100 grains. 
Resistance per mile 
„ per yard 

„ per mil. foot 

0*00000066788 

0*00000169639 

0*150158 

0*042317 -f area inn" 
0 000024044 4- ,, 
10*2044 

0*000000681327 

0*00000173054 

0*153181 

0*0431689 -T-area in □" 
0*00002452774- ,, 
10*4099 


Weight per mile . • . . . 2*0350 x area in 

ff )) yaid . • . • 11*5625 x 

The following table refers to copper cables with a lay = 20 times 
the pitch diameter. 


Table XVI. 


Cable. 

Resistance in Standard 
Ohms. 

Weight. 

3-Strand 

0*33742 xr 

3*03678x14; 

4- „ 

0*253065 xr 

4*04904 X w 

7- „ 

0*1443557 xr 

7*07356 X la 

1*2- ,, 

0*084355 xr 

12*1471 X14; 

19- ,, 

0*0532424 x r 

19*2207 xto 

37- „ 

0*0273493 xr 

37*4414 xw 

61- „ 

0*0165911 xr 

61*7356 xw 

91- „ 

0*0111222 xr 

92*1034 xw 


where r = the resistance of each wire, 
and w= ,, weight 

The resistance of a cable being equal to the resistance in parallel of the wires, 

T T 



















642 


ELECTRICAL ENGINEERING TESTING 


International Standards of Resistance for 

Copper. 

Extract from the British Engineering Standards Association 

Report^ No. 7, July 1919. 

1. The following standards, fixed by the International Electro- 
Technical Commission, have been taken as normal values for 
standard annealed copper :— 

(i) At a temperature of 20° C. the resistance of a wire of 

standard annealed copper, one metre in length and of 
a uniform section of one square millimetre, is 1/58 ohm 
(0-017241 . . . ohm). 

(ii) At a temperature of 20° C. the density of standard 

annealed copper is 8'89 grammes per cubic centimetre. 

(iii) At a temperature of 20° C. the “ constant-mass ” 

perature coefficient of resistance of standard annealed 
copper, measured between two potential points rigidly 
fixed to the wire, is— 

0-00393 = 1/254-45 . . . per degree Cent. 

(iv) As a consequence it follows from (i) and (ii) that at a 

temperature of 20° C. the resistance of a wire of 
standard annealed copper of uniform section, one metre 
in length and weighing one gramme, is— 

(1/58) X 8-89 = 0 15328 ohm. 

Coefficient of Linear Expansion of Standard Annealed 

Copper. 

2. The coefficient of linear expansion of standard annealed 
copper between 60° F. (15'6° C.) and 68° F. (20° C.) has been 
taken as— 

0-00000944 per 1° F. (0-0000170 per 1° C.). 

Density of Standard Annealed Copper at 60° F. 

3. The density of standard annealed copper at a temperature 
of 60° F. has been taken as 8-892015, and the weight per one 
cubic foot of copper as 555-1108 lbs. 

Resistance of a Solid Conductor at 60° F. 

4. For the purpose of calculating the tables, the resistance of 
a solid conductor of standard annealed copper at 60° F., 1000 
yards in length and of uniform cross-sectional area of one square 
inch, has been taken as 0-0240079 ohm. 


ELEGTBICAL ENGINEERING TESTING 


643 


Table XVII. 

Relation between E.M.F. and Temperature op the Clark 
Standard Cell—made according to Regulations, op the 
Carhart-Clark, and op the Weston Cadmium Standard 
Cells (see pp. 13 and 17). 


Temper¬ 

ature 

°C. 

E.M.P. in Legal Volts. 

Temper¬ 

ature 

°C. 

E.M.P. in Legal Volts. 

Clark. 

Carliart- 

Clark. 

Weston 

Cadmium. 

Clark. 

Carhart- 

Clark. 

Weston 

Cadmium. 

4 

1'44C1 

1-4395 

1-01895 

16 

1*4329 

1-4335 

1-01846 

5 

1N450 

1-4390 

91 

17 

1-4318 

1-4330 

42 

6 

1*4439 

1-4385 

87 

18 

1-4307 

1-4325 

3S 

7 

1*44'28 

1-4380 

83 

19 

1-4296 

1-4320 

34 

8 

1*4417 

1-4375 

79 

20 

1-4285 

1-4315 

30 

9 

1*4406 

1-4370 

75 

21 

1-4274 

1-4310 

26 

10 

1*4395 

1-4365 

71 

22 

1-4263 

1-4305 

22 

11 

1*4384 

1-4360 

66 

23 

1-4252 

1-4300 

18 

12 

1*4373 

1-4355 

62 

24 

1-4241 

1-4295 

14 

13 

1*4362 

1-4350 

58 

25 

1-4230 

1-4290 

10 

14 

1*4351 

1-4345 

54 

26 

1-4219 

1-4285 

06 

15 

1-4340 

1-4340 

1*01850 

27 

1-4208 

1-4280 

1-01802 


. Table XVIIT. 

Relation between Practical and C.G.S. (Absolute) Units. 




Absolute C.G.S. units. 

Dimensions. 


Practical 

units. 

Electro¬ 

mag¬ 

netic. 

Electrostatic. 

Electro¬ 

static. 

Electro¬ 

magnetic. 

Quantity . 

Current 

Potential . 

Resistance. 

Cai)acity . 
Self-induction . 
Mutual induction 

Power 

Work 

Induction . 

1 coulomb 

1 ampere 

1 volt 

1 ohm 

1 farad 

1 secohm 

1 secohm 

1 watt 

1 joule 

1 webcr 

10“^ 

10“^ 

10® 

10® 

10"^ 

109 

109 

107 

107 

103 

i;xl0“^=3xl0® 
•j;Xl0“^=3Xl0® 2 

108_j.^ = J XIO" 
^\xl0»=5xl0-ii 

i;9xl0"^=9xl0^^ 

v=3X 10^® 

mMt“^ 

m^l^t"® 

m^l^t" 

l"^t 

L 

M^L^ 

m^l^t-^ 

mMt-® 

LT~^ 

L“V 

L 

L 

ml^t-^ 

Ml2t-2 


velocity of light = 3x10^° cms. per second. 








































644 


ELECTRICAL ENGINEERING TESTING 


Table XIX. 

ELECTRO-CnEMICAL EQUIVALENTS, SPECIFIC GRAVITIES, ETC. 


Metal. 

Atomic 

weight. 

Chemical 
equivalent 
_ at. wt. 
valency 

Aluminium . 

270 

9-0 

Copper (monad) . 

63-1 

63-1 

„ (dyad) . 

63-1 

81-6 

Gold . 

196-7 

65-6 

Iron (dyad) . 

56 

28 

Lead . 

206-4 

103-2 

Nickel. . 

68-6 

29-3 

Silver . 

108 

108 

Tin (dyad) . 

117-8 

68-9 

Zinc 

65 

32-5 

Hydrogen . 

1 

1 


Electro-chemical 
equivalent, 
grams per 
coulomb. 

Weight in 
grams per 
hour deposited 
by 1 amp. 

Specific 
gravity 
in grams 
per 

cub. cm. 

0-00009317 

0-3354 

2-67 

0-00065735 

2-3665 

8-912 

0-00032867 

1-1832 

8-912 

0-00067806 

2-4410 

19-3 

0-00028986 

1-0435 

7-85 

0-0010714 

3-8571 

11-4 

0-00030538 

1-0994 

8-5 

0-0011180 

4-0249 

10-57 

0-00061077 

2-1988 

7-3 

0-00033644 

1-2112 

7-15 

0-000010352 

0-03738 

0-0000896 


Table XX. 

Temperature Coefficients and Specific Resistances of Pure 
Metals and Alloys, determined by Professors J. A. Fleming 
AND J. Dewar. 


Metals, 
pure, soft, 
and 

annealed. 

Specific 
resistance, 
p, in 

microhms, 
per c.c. 
at 0'0. 

Mean 

temperature 
coefficient, 
a, between 
O’andlOO'C. 

Alloys, 

usual proportions. , 

Specific 
resistance, 
p, in 

microhms, 
per c.c. 
at 0° C- 

Temperature 
coefficient, 
a, at 15* C. 

Platinum . 

10-917 

0-003609 

Platinum-silver . 

81-582 

0-000243 

Gold . 

2-197 

0-003770 

„ -iridium 

30-896 

0-000822 

Palladium . 

10-219 

0-003540 

,, -rhodium 

21-142 

0-00143 

Silver . 

1-468 

0-004000 

Gold-silver 

6-280 

0-00124 

Cojiper 

1-561 

0-004280 

Manganese steel . 

67-148 

0-00127 

Aluminium. 

2-665 

0-004350 

Nickel steel . 

29-452 

0-00201 

Iron 1 . 

9-065 

0-006250 

German silver . 

29-982 

0-000273 

Nickel 

12-323 

0-006220 

Platinoid 

41-731 

0-000310 

Tin . . 

13-048 

0-004400 

Manganin . . 

46-678 

-0-00000 

Magnesium. 

4-355 

0-003810 

Silverene 

2-064 

0-00285 

Zinc . 

5-751 

0-004060 

Aluminium-silver 

4-641 

0-00238 

Cadmium . 

10-023 

0-004190 

„ -copper 

2-904 

0-00381 

Lead . 

20-380 

0-004110 

,, -bronze * . 

12-300 

0-0010 

Thallium . 

17-633 

0-003980 

Reostene* . 

76-468 

000110 

Mercury 

94-070 

0-000720 

Brass * . . 

7-2 

0-0010 

Bismuth* . 

119-160 

0-00420 

Nickelin * . 

38-50 

_ 

Cobalt • 

9-71 

0-00330 

Carbon * retort . 

67000 


Tantalum *. 
Tungsten* . 

14-6 

6-0 

0-00330 

0 0051 

,, arc light Carr6 . 

,, glow lamp Edi- 

7000 

-0-0005 

Osmium* ., 

9-5 


swan.... 
Phosphor bronze* 
(commercial) . 
Eureka * . . . 

Constantan * 

Nickel Chrome . 

4000 

8-479 

47-48 

100 

-0-00054 

0-00064 
0-000022 
/-0-00004 to 
\+ 0-00001 
-00042 


» Ai'proxiuiately pure. ? Not determined by Fleming and Dewar. 





























ELEGTBIGAL ENGINEERING TESTING 


645 


Table XXI. 

Approximate Specific Kesistance op the Commoner Liquids, and 
WHICH Diminishes with Increase op Temperature about 
1*5% PER 1° C. 


• 

Solution. 

Specific Resistance 
(Legal ohms per c.c.). 

At a temper¬ 
ature (Degrees 
Cent.). 

Sulphuric acid, 5% acid, sp. gr. 1'033 

4*82 

18 

„ „ 10% „ „ 1*070 

2*84 


„ ,, 20% „ „ 1*1414 

1*64 

99 

„ „ 25% „ „ 1170 

0*99 

99 

„ „ 30% „ „ 1*220 

1*36 

99 

„ » 40% „ „ 1*310 

1*48 

99 

Common salt (ordinary)—saturated solution 

5*10 

99 

Zinc sulphate ,, ,, ,, 

20*3 

99 

Copper ,, „ ,, ,, 

300 

99 

Sal-ammoniac solution, sp. gr. 1*07 

5*50 

99 

Water 

0-3x106 

11 

$9 

120x106 

76 


Table XXII. 

Comparative Data for H.C. Copper and Aluminium. 



Copper. 

Aluminium. 

Conductivity (Electrical). 

Tensile Strength (unannealed). 

Specific Gravity. 

Sectional Area for Equal Conductivity 

Diameter >> >> »> ... 

Weight M )) >» ... 

Temperature Co efficient per 1“ C. . . . 

100 

25-30 tons 
1200° 0. 

8-9 

1 

1 

1 

0-C0)28 

61-5 

12-15 tons 
700° G. 

2-7 

1-66 

1.28 

0-495 

0-0032 


























646 


ELECTRICAL ENGINEERING TESTING 


Table XXIII. 

Eureka Resistance Material 


A Cupro-Nickel Alloy (supplied by the London Electric 
Wire Co. and Smith’s Ltd.), prepared with great care 
TO secure a Non-corrodible and Stable Alloy. 


Temperature Co-efficient 
Specific Resistance . . 

„ Gravity , . . 

Melting-Point . . . 

Comparative Resistance 
Tensile Strength . . 

Weight per cubic inch . 


0'000022 per deg. C. 

48 Microhms per cm. cube. 

So"0. 

28 times copper. 

40 tons per sq. inch. 

0-32 lb. 


Resistance and carrying capacity for open spirals^' of Eureka 
wire in air^ well ventilated with free radiation. 


Gauge 

S.W.G. 

Diameter. 

Approx. Amps giving temperatun s oJ 

Res stance in 
Standard Ohms 
per 1000 yds. at 
60° F. (15-5° C.) 

Inches. 

m/m. 

100* 0. 

2(0* C. 

800° C. 

6 

•192 

4-877 




23-8 

7 

•176 

4-470 




27-7 

8 

•160 

4-06 

33-0 

52 

68-5 

83-5 

9 

•144 

3-65 

26-0 

43 

50 

41-3 

10 

•128 

325 

22 8 

36 

41-5 

52-3 

11 

•116 

2-94 

190 

30 

85-5 

63-7 

12 

•104 

2-64 

16-8 

24 

29-5 

79-3 

13 

•092 

2-33 

12-7 

20 

24-2 

101-3 

14 

•080 

2-03 

9-5 

15 

19-5 

133-9 

15 

•072 

1-82 

7-4 

12-6 

16-8 

165-3 

16 

•064 

1-62 

6 0 

10-4 

14-3 

209-4 

17 

•056 

1-42 

5 3 

8-8 

11-3 

273-3 

18 

•048 

1-21 

4-3 

7-0 

9 1 

8718 

19 

•040 

1-01 

3-7 

5-5 

6-8 

6.35-6 

20 

•036 

•914 

3-0 

4-7 

5-9 

661-3 

21 

•032 

•812 

2-8 

4-0 

5-0 

837-2 

22 

•028 

•711 

2-2 

3-2 

4-1 

1093 

23 

•024 

•609 

1-3 

2-6 

3-3 

1457 

24 

•022 

•558 

1-5 

2-3 

2-8 

1770 

25 

•020 

•508 

1-25 

2 0 

2-5 

2142 

26 

•018 

•457 

ro 

1-63 

2 1 

2645 

27 

•0164 

•416 

•9 

1-47 

1-9 

3186 

28 

•0148 

•375 

•76 

1-37 

1-58 

3914 

29 

•0136 

•345 

•68 

1-15 

1-47 

4634 

30 

•0124 

•311 

•59 

1-0 

1-25 

5575 

31 

■0116 

•294 

•52 

•9 

1-05 

6370 

32 

•0108 

•274 

•47 

•81 

•95 

7350 

33 

•010 

•263 

•42 

•74 

•85 

8571 

34 

•0092 

•233 

•37 

•64 

•75 

10128 

35 

•0084 

•213 

■33 

•66 

•65 

12149 

36 

•0076 

•193 

•28 

•48 

•57 

14840 

37 

•0068 

•172 

•26 

•43 

•51 

18536 

38 

•006 

•152 

•19 

•31 

•40 

23808 

39 

•0052 

•132 

•16 

•26 

•31 

31696 

40 

•0048 

•121 

•15 

•24 

•28 

37184 

41 

•0044 

•111 

•14 

•21 

•26 

44268 

42 

•004 

•101 

•13 

•18 

•23 

53564 

43 

•0086 

•091 

•11 

•17 

•20 

66136 

44 

•0032 

•081 

•10 

•14 

•17 

83664 

45 

•0028 

•071 

•08 

‘13 

•15 

108648 

46 

•0024 

•061 

•07 

•10 

•12 

148764 

47 

•002 

•050 

•05 

•08 

•10 

214284 






















ULECTEIGAL ENGINEERING TESTING 


647 


Table XXIY. 


Nickel Chrome Resistance Material 

A High Resistance Alloy (supplied by the London Electric 

Wire Co. and Smith’s Ltd.). 


Temperature Co-efficient 
Specific Resistance . . 

,, Gravity. . . 

Melting-Point . . . 
Comparative Resistance 
Tensile Strength . . 

Weight per cubic inch . 


•00042 per deg. 0. 

100 Microhms per cm. cube. 
8-15. 

1550° C. 

68 times copper. 

47*12 tons per sq. Inch. 

•29 lb. 


Resistance and carrying capacity of Nickel Chrome Wire. 


Size 

S.W.G. 


16 

17 

18 

19 

20 

21 

22 

23 

24 

25 

26 

27 

28 

29 

30 

31 

32 

33 

34 

35 

36 

37 

38 

39 
-50 


Approximate Resistance in Standard Ohms 

Approximate Amperes givins 

per 1000 yds. at temperatures of 

temperatures of 

200° C. 

400° C, 

d 

o 

o 

o 

200° C. 

400° C. 

600° C. 

452 

494 

638 

7*1 

12 

18 

591 

646 

703 

6-0 

9-6 

14 

802 

8T9 

957 

4-3 

7-7 

11 

1154 

1266 

1378 

3-7 

5-7 

8-4 

1426 

1590 

1700 

3-3 

4-7 

6-S 

1809 

1978 

2151 

2-7 

4-2 

6-2 

2360 

258? 

28-20 

2-2 

3-5 

51 

3-2;-i7 

3535 

3860 

1-8 

2-8 

4-1 

3828 

4187 

4555 

1-6 

2-4 

3-3 

4732 

5061 

. 5505 

1-4 

2-1 

3-1 

5720 

6250 

6870 

1-1 

1-9 

2-6 

6890 

7535 

8400 

1-0 

1-6 

2-4 

8460 

9250 

10070 

•93 

1-4 

2-0 

10000 

10950 

11920 

•78 

1-3 

1-8 

12040 

13170 

14320 

•68 

11 

1-6 

13760 

15040 

16370 

•61 

•88 

1-3 

15880 

17360 

18900 

•55 

•80 

1-2 

18530 

20250 

22050 

•50 

•72 

11 

21880 

23920 

26100 

•43 

•63 

•93 

26250 

28700 

31500 

•37 

•56 

•83 

32200 

35070 

38380 

•32 

•49 

•72 

40100 

43800 

49000 

•29 

•43 

•63 

61400 

56300 

61270 

•21 

•34 

•49 

68500 

74900 

81500 

•17 

•26 

•39 

80-200 

87900 

95700 

•10 

•24 

•35 





































648 


ELEGTRIOAL ENGINEERING TESTING 


Table XXV. 

The following Table shows the Current that will give certain 
RISES IN Temperature in various gauges op Eeostene Resist¬ 
ance Material, and also the Resistances and Watts ab¬ 
sorbed PER YARD IN EACH CASE (W. T. GlOVER AND Co.). 


S. W. G. 

Ohms 

per 

yard at 
15-5'’ C. 

50“ C. rise in temper¬ 
ature. 

100“ C. rise in 
temperature. 

150“ 0. rise in 
temperature. 

200“ C. rise in tem¬ 
perature. 

Ohms per yard at 
65-5* C. 

Amperes giving a 
rise of 50“ C. 

Watts consumed 
per yard. 

Ohms per yard at 
115*5“ C. 

Amperes giving a 
rise of 100“ C. 

Watts consumed 
per yard. 

Ohms per yard at 
165-5“ 0. 

Amperes giving a 

rise of 150“ C. 

Watts consumed 

per yard. 

Ohms per yard at 

215-5“ C. 

Amperes giving a 

rise of 200“ C. 

Watts consumed 

per yard. 

8 

•0541 

•0571 

20-1 

23-1 

•0601 

33-0 

65-5 

•0631 

39 

96 

•0661 

53 

186 

9 

•0669 

•0706 

17-8 

22-4 

•0743 

28-1 

58-6 

•0780 

34 

90 

•0817 

43 

151 

10 

•0845 

•0890 

15-1 

20-3 

•0938 

22-8 

48-75 

•0984 

27-8 

76-00 

•103 

35-6 

131 

11 

•1024 

•1080 

13-2 

18-8 

•1137 

19-6 

43-70 

•119 

23-7 

66 7 

•125 

30-5 

116 

12 

•1290 

•136 

11-3 

17-4 

•143 

16-6 

39 35 

•150 

20-2 

61-2 

•157 

26'0 

106 

18 

•1646 

•174 

9-4 

15-4 

•183 

13-7 

34-35 

•192 

16-6 

52-8 

•201 

217 

95 

14 

•2180 

•230 

7-6 

13-3 

•242 

11-6 

32 55 

•254 

13-9 

49-0 

•266 

18-0 

86 

15 

•2670 

•282 

6-7 

12-7 

•296 

10-0 

29-60 

•311 

12-1 

45-5 

•326 

16-0 

83 

16 

•3382 

•357 

6-7 

11-6 

•377 

8-60 

27-90 

•394 

10-5 

43-4 

•413 

13-4 

74 

17 

•4406 

•465 

4-7 

10-3 

•489 

7-20 

25-4 

•513 

8-8 

39-7 

•547 

11-3 

70 

18 

•600S 

•634 

4-0 

10-15 

•667 

5-93 

23-5 

•700 

7-35 

37-8 

•733 

9-27 

63 

19 

•8677 

•915 

3-2 

9-38 

•963 

4-80 

22-2 

1-01 

5-91 

35-3 

1-06 

7-45 

59 

20 

1-06S 

1-13 

2*84 

912 

1-19 

4-25 

21-5 

1-24 

5-25 

34 2 

1-30 

6-66 

58 

21 

1-3528 

1-43 

2-6 

8-94 

1-50 

3-75 

21-1 

1-69 

4-61 

33-8 

1-65 

5-80 

56 

22 

1-7666 

1-86 

2-15 

8-60 

1-96 

3-30 

21-4 

2-06 

4-03 

33-5 

2-16 

5-07 

55-5 

23 

2-4030 

2-54 

1-83 

8-50 

2-67 

2-80 20-9 

2-80 

3-44 

33-2 

2-93 

4-33 

55-0 

24 

2-8658 

3-02 

1-62 

7-93 

3-18 

2-53 20-4 

3-34 

3-12 

32-5 

3-50 

3-96 

54-8 

25 

3-4665 

3-66 

1-49 

8-13 

3-85 

2-29 20-2 

1 

4-04 

2-82 

32-2 

4-23 

3-67 

54-0 

26 

4-2764 

4-51 

1-32 

7-86 

4-75 

2-05 20-0 

4-98 

2-52 

31-6 

5-22 

8-19 

53-2 

27 

5-1531 

5-44 

1-2 

7-83 

5-72 

1-84 19-4 

6-00 

2-29 

31-4 

6-29 

2-90 

52-8 

28 

6-3279 

6-68 

1-07 

7-65 

7-02 

1-65 

19-1 

7-37 

2-05 

31-0 

7-73 

2-59 

52-3 

29 

7-4938 

7-91 

•98 

7-59 

8-32 

1-51 

19-0 

8-73 

1-87 

30-4 

9-14 

2-36 

50-8 

30 

9-01125 

9-61 

a 

GO 

7-61 

10-00 

1-36 

18-5 

10-50 

1-69 

29-9 

11-00 

2-15 

50-8 



































ELEGTRIGAL ENGINEERING TESTING 


649 


Table XXVI. 


Eelation between gauges op covered Manganin wire and currents 


GIVING A rise op TEMPERATURE OP 100° C. ABOVE THE OUTSIDE 
Air (W. T. Glover and Co.). 


No. B.W.G. 

Amperes. 

No. B.W.G. 

Amperes. 

No. B.W.G. 

Amperes. 

No. B.W.G. 

Amperes. 

2 

60-6 

12 

12-0 

22 

1-57 

32 

0-386 

3 

49-0 

13 

9*8 

23 

1-32 

83 

0-335 

4 

39 0 

14 

7-97 

24 

1-093 

84 

0-31 

6 

34-6 

15 

6-47 

25 

0-952 

35 

0-272 

6 

30-6 

16 

5-55 

26 

0-807 

36 

0-235 

7 

25-5 

17 

4-68 

27 

0-677* 

37 

0-209 

8 

22-5 

18 

3-63 

28 

0-555 

38 

0 188 

9 

19-0 

19 

2-90 

29 

0-497 

39 

0-167 

10 

16-4 

20 

2-2 

30 

0-440 

40 

0148 

11 

13-9 

21 

1-9 

31 

0-410 




Note.— For bare wires in air Mr. L. B. Atkinson allows 1 sq. In. of total external 
surface per Watt to be dissipated for a temperature of 160* CL 


Table XXVII. 

Showing the Currents that will Produce 212'’ F. (100° 0.) rise in 
Temperature above the Surrounding Air for bare Manganin 
wire stretched horizontally and freely exposed to the Air 
(W. T. Glover and Co.). 


S. W. G . . . 

No. 

8 

9 

10 

11 

12 

13 

14 

Amperes. . . 


60 

50 

40 

35 

80 

25 

20 

S.W. G. . . . 

No. 

15 

16 

17 

18 

19 

20 

21 

Amperes. . . 


15 

12 

10 

9 

8 

6 

5 


Note.—I f placed vertically or coiled, an allowance must be made. 






















650 


ELECTRICAL ENGINEERING TESTING 


Table XXYIII. 


Densities op Dry Air in lbs. per cubio foot at different 

Temperatures and Pressures calculated by the Rela- 

, , r 0-001293 H 1 

TION (lbs. per cubic EOOT) = | ^ X X 

62*43, WHERE T = Temp. in ° C. and H = Pressure in mm. 
OF Mercury at 0°C., Lat. 45°, ^= 980*62. 0-001293 = 
Density in grams per c.c. at 0° C. and 760 mm. Pressure 
OF Mercury, 1 gram per c.c. = 62*43 'Lbs. per cubic foot. 


Temp. 


Barometric Pressure in Millimetres of Mercury 

= H. 


T“ C. 

710 ram. 

720* 

730 

740 

750 

760 

770 

780 


27*95 in. 

28*34 

28*74 

29*13 

29*52 

29*92 

30*31 

30*70 

0 

0*07541 

0*07647 

0*07754 

0*07859 

0*07968 

0*08072 

0-08179 

0*08285 

5 

7406 

7509 

7617 

7716 

7821 

7929 

8030 

8134 

10 

7274 

7381 

7480 

7578 

7685 

7784 

7893 

7991 

15 

7148 

7249 

7348 

7447 

7554 

7647 

7754 

7847 

20 

7023 

7124 

7223 

7323 

7425 

7521 

7617 

7716 

25 

6904 

7008 

7098 

7197 

7300 

7392 

7492 

7581 

30 

6791 

6885 

6984 

7079 

7173 

7274 

7367 

7461 


Table XXIX. 

Comparison op Wire Gauges in Common Use. 


No. 

S.W.G. 

inch. 

B.W.G. 

inch. 

B.& S. 

inch. 

No. 

S.W.G. 

inch. 

B W.G. 

inch. 

B. &S. 

inch. 

No. 

S.W.G. 

inch. 

B.W.G. 

inch. 

B.&S. 

inch. 

4/0 

.400 

•454 

•4600 

15 

•072 

*072 

•0571 

33 

*0100 

•008 

*0071 

3/0 

*372 

•425 

•4096 

16 

•064 

*065 

•0508 

34 

*0092 

♦007 

*0063 

2/0 

*348 

•380 

•3648 

17 

•056 

*058 

•0453 

35 

*0084 

•005 

•0056 

0 

*324 

•340 

•3249 

18 

•048 

*049 

•0403 

36 

*0076 

*004 

*0050 

1 

*300 

•300 

*2893 

19 

•040 

*042 

•0359 

37 

*0068 


*0045 

2 

*276 

•284 

*2576 

20 

•036 

*035 

*0320 

38 

*0060 


*0040 

3 

•252 

•259 

*2294 

21 

•032 

*032 

*0285 

39 

*0052 


*0035 

4 

*232 

•230 

*2043 

22 

•028 

*028 

*0253 

40 

*0048 


*0031 

5 

*212 

•220 

•1819 

23 

•024 

-025 

•0226 

41 

*0044 



6 

*192 

•203 

•1620 

24 

•022 

•022 

•0201 

42 

*0040 



7 

*176 

•180 

*1443 

25 

•020 

•020 

*0179 

43 

•0036 



8 

*160 

•165 

•1285 

26 

•018 

*018 

•0159 

44 

*0032 



9 

•144 

•148 

•1144 

27 

•0164 

*016 

•0142 

45 

•0028 



10 

•128 

•134 

•1019 

28 

•0148 

•014 

*0126 

46 

•0024 



11 

•116 

•120 

•0907 

29 

•0136 

•013 

•0113 

47 

•0020 



12 

•104 

•109 

•080S 

30 

•0124 

*012 

•0100 

48 

•0016 



13 

•092 

•095 

•0720 

31 

*0116 

*010 

*0089 

49 

•0012 



14 

•080 

•083 

*0641 

32 

*0108 

*009 

*0079 

50 

•0010 












































ELEGTBIGAL ENGINEERING TESTING 


651 


Photometer Bench. 


Table XXX. 

Ratios of Squares op Distances from Screen to Sources op 
Light for different Distances (D) between the latter (for 
facilitating Calculations op Candle Powers). 


(D - d)2 

O.P. to be determined «= O.P. of standard x —— where (d) = distance between 
screen and standard. 

Note. —Intermediate values may very approximately be found by proportion, but shoxUd 

(D - d)2 

be obtained from a curve between d and —^ 2 — when required more accurately. 


(D - d)2 
d2 

200 

800 

II 0 

600 

600 

(2) - (i2) 

200 

300 

D = 

400 

500 

600 

d 

d 

d 

d 

d 

d 

d 

d 

d 

d 

81-00 

20-0 

30-0 

40 

50-0 

60-0 

17-04 

39-0 

58-5 

78 

97-5 

117-0 

76-67 

0-5 

0-75 

1 

1-25 

1-5 

16-51 

9-5 

9-25 

9 

8-75 

8-5 

72-68 

1-0 

1-5 

2 

2-50 

3-0 

16-00 

40-0 

60-0 

80 

100-0 

120-0 

68-93 

1-5 

2-25 

8 

3-75 

4-5 

15-51 

0-5 

0-75 

1 

1-25 

1-5 

65-46 

2-0 

3-0 

4 

5-0 

6-0 

15-04 

1-0 

1-5 

2 

2-5 

3-0 

62-23 

2-5 

3-75 

5 

6-25 

7-5 

14-59 

1-5 

2-25 

3 

3-75 

4-5 

69-24 

8-0 

4-5 

6 

7-6 

90 

14-15 

2-0 

8-0 

4 

6-0 

6-0 

66-41 

8-5 

5-25 

7 

8-75 

70-5 

13-74 

2-5 

3-75 

5 

6-25 

7-5 

63-77 

4-0 

6-0 

8 

60-0 

2-0 

13-32 

8-0 

4-5 

6 

7-5 

9-0 

61-31 

4-6 

6-75 

9 

1-25 

8-6 

12-94 

3-5 

6-25 

7 

8-75 

130-5 

49-00 

6-0 

7-5 

60 

2-5 

6-0 

12-57 

4-0 

6-0 

8 

110-0 

2-0 

46-81 

5-5 

8-25 

1 

3-75 

6-5 

12-21 

4-6 

6-75 

9 

1-25 

8-5 

44-78 

6-0 

9-0 

2 

5-0 

8-0 

11-87 

6-0 

7-5 

90 

2-5 

6-0 

42-85 

6-5 

9-75 

3 

6-25 

9-6 

11-53 

5-5 

8-25 

1 

3-75 

6-5 

41-04 

7-0 

40-5 

4 

7-5 

81-0 

11-21 

6-0 

9-0 

2 

6-0 

8-0 

89-33 

7-5 

1-25 

5 

8-75 

2-5 

10-89 

6-5 

9-75 

8 

6-25 

9-5 

37-73 

8-0 

2-0 

6 

70-0 

4-0 

10-59 

7-0 

70-5 

4 

7-5 

141-0 

86-21 

8-5 

2-75 

7 

1-25 

5-5 

10-30 

7-5 

1-25 

5 

8-75 

2 5 

34-77 

9-0 

3-5 

8 

2-5 

7-0 

10-02 

8-0 

2-0 

6 

120-0 

4-0 

33-40 

9-5 

4-25 

9 

8-75 

8-5 

9-754 

8-5 

2-75 

7 

1-25 

5-5 

32-11 

80-0 

5-0 

60 

6-0 

90-0 

9-497 

9-0 

3-5 

8 

2-5 

7-0 

80-89 

0-5 

6-75 

1 

6-25 

1-5 

9-247 

9-5 

4-25 

9 

3-75 

8-5 

29-72 

1-0 

6-5 

2 

7-5 

3-0 

9-00 

50-0 

5-0 

100 

5-0 

150-0 

28-62 

1-5 

7-25 

3 

8-76 

4-5 

8-535 

1 

6-5 

2 

7-5 

3 

27-56 

2-0 

8-0 

4 

80-0 

6-0 

8-102 

2 

8-0 

4 

130-0 

6 

26-56 

2-5 

8-75 

5 

1-25 

7-5 

7-691 

8 

9-5 

6 

2-5 

9 

25-61 

8-0 

9-5 

6 

2-5 

9-0 

7-300 

4 

81-0 

8 

5-0 

162 

24-69 

8-5 

60-25 

7 

3-75 

100-5 

6-950 

6 

2-5 

110 

7-5 

5 

23-83 

4-0 

1-0 

8 

5-0 

2-0 

6-613 

6 

4-0 

2 

140-0 

8 

23-01 

4-5 

1-75 

9 

6-25 

3-5 

6-295 

7 

6-5 

4 

2-5 

171 

22-22 

5-0 

2-5 

70 

7-5 

5-0 

5-985 

8 

7-0 

6 

5-0 

4 

21-47 

6-5 

3-25 

1 

8-75 

6-6 

6-710 

9 

8-5 

8 

7-5 

7 

20-75 

6-0 

4-0 

2 

90-0 

8-0 

5-44 

60 

90-0 

120 

150-0 

180 

20-06 

6-5 

4-75 

3 

1-25 

9-5 

5-193 

1 

1-5 

2 

2-5 

8 

19-41 

7-0 

5-6 

4 

2-5 

111-0 

4-909 

2 

3-0 

4 

6-0 

6 

18-78 

7-5 

6-25 

5 

3-75 

2-5 

4-730 

3 

4-5 

6 

7-5 

9 

18-17 

8-0 

7-0 

6 

5-0 

4-0 

4-51 

4 

6-0 

8 

160-0 

192 

17-60 

8-5 

7-75 

7 

6-25 

5 5 

4-313 

5 

7-5 

130 

2-5 

5 
































652 


ELECTBIGAL ENGINEEIUNG TESTING 


(D - dV 

200 

800 

D = 

400 

500 

600 

(D - dl2 

200 

300 

2) = 

400 

500 

600 

ili 

d 

d 

d 

d 

d 

d2 

d 

d 

d 

d 

d 

4123 

66-0 

99-0 

132 

165-0 

198-0 

0-0918 

153-5 

230-25 

307 

383-76 

460-6 

3-941 

7 

100-5 

4 

7-5 

201 

0-0892 

4-0 

1-0 

8 

5-0 

2-0 

8-769 

8 

2-0 

6 

170-0 

4 

•0867 

4-5 

1-75 

9 

6-25 

3-5 

3-606 

9 

3-5 

8 

2-6 

7 

•0842 

5-0 

2-5 

310 

7-5 

5-0 

3-45 

70-0 

5-0 

140 

6 0 

210 

•0819 

5-5 

3-25 

1 

8-75 

6’5 

3-091 

2-5 

8-75 

145 

181-25 

7-5 

0-0795 

6-0 

4-0 

2 

890-0 

8-0 

2-777 

5-0 

112-5 

150 

7-5 

225-0 

•0773 

6-5 

4-75 

8 

1-25 

9-5 

2-500 

7-5 

6-25 

155 

193-75 

232-5 

•0751 

7-0 

5-5 

4 

2-5 

471-0 

2-25 

80-0 

120-0 

160 

200-0 

240-0 

•0728 

7-5 

6-25 

5 

8-75 

2-5 

2-029 

2-5 

8-75 

165 

206-25 

7-5 

•0707 

8-0 

7-0 

6 

5-0 

4-0 

1-831 

5-0 

7-5 

170 

212-5 

255-0 

0-0685 

8-5 

7-75 

7 

6-25 

5-5 

1-654 

7-5 

131-25 

175 

218-75 

262-5 

•0665 

9-0 

8-5 

8 

7-5 

7-0 

1-49 

90-0 

5-0 

180 

225-0 

270-0 

•0645 

9-5 

9-25 

9 

8-75 

8-5 

1-350 

2-5 

8-75 

185 

231-25 

7-5 

•0625 

160-0 

240-0 

320 

400-0 

480-0 

1-222 

6-0 

142-5 

190 

237-5 

285-0 

•0606 

0-5 

0-75 

1 

1-25 

1-5 

1-106 

7-5 

6-25 

195 

243-75 

292-5 

0-0587 

1-0 

1-5 

2 

2-5 

3-0 

1 

100-0 

150-0 

200 

250-0 

300-0 

•0568 

1-5 

2-25 

3 

8-75 

4-5 

0-9045 

2-5 

3-75 

205 

256-25 

7-5 

•0551 

2-0 

3-0 

4 

5-0 

6-0 

0-8185 

5-0 

7-6 

210 

262-5 

315-0 

•0533 

2-5 

3-75 

5 

6-25 

7-5 

•7407 

7-5 

161-25 

215 

268-75 

322-5 

•0515 

3-0 

4-5 

6 

7-5 

9-0 

0-6711 

110-0 

6-0 

220 

275-0 

330-0 

0-0499 

3-5 

5-25 

7 

8-75 

490-5 

•6046 

2-5 

8-75 

225 

281-25 

7-6 

•0482 

4-0 

6-0 

8 

410-0 

2-0 

0-5461 

6-0 

172-5 

230 

287-5 

345-0 

•0466 

4-5 

6-75 

9 

1 25 

3-5 

•4929 

7-5 

6-25 

235 

293-75 

352-5 

•0450 

5-0 

7-5 

830 

2-5 

6-0 

0-44 

120-0 

180-0 

240 

300-0 

360-0 

•0435 

6-5 

8-25 

1 

8-75 

6-5 

•400 

2-5 

3-75 

245 

306-25 

7-5 

•0420 

6 0 

9-0 

2 

5-0 

8-0 

0-3601 

6-0 

7-5 

250 

312-5 

375-0 

•0405 

6-5 

9-75 

3 

6-25 

9-5 

•3236 

7-5 

191-25 

255 

8-75 

382-5 

0-0391 

7-0 

250-5 

4 

7-5 

501-0 

0-29 

130-0 

5-0 

260 

325-0 

390-0 

•0377 

7-5 

1-25 

5 

8-75 

2-6 

•2773 

1 

6-5 

2 

7-5 

8 

•0363 

8-0 

2-0 

6 

420-0 

40 

•2653 

2 

8-0 

4 

330-0 

6 

•0349 

8-5 

2-75 

7 

1-25 

5-5 

•2537 

8 

9-5 

6 

2-5 

9 

•0336 

9-0 

3-5 

8 

2-5 

7-0 

•2425 

4 

201-0 

8 

5-0 

402 

•0324 

9-5 

4-25 

9 

8-75 

8-5 

•2318 

5 

2-5 

270 

7-5 

5 

•03114 

170-0 

255-0 

340 

5-0 

510-0 

•2217 

6 

4-0 

2 

340-0 

8 

0-0299 

0-5 

5-75 

1 

6-25 

1-5 

•2114 

7 

5-5 

4 

2-5 

411 

•0288 

1-0 

6-6 

2 

7-6 

3-0 

•2037 

8 

7-0 

6 

5-0 

4 

•0276 

1-5 

7-25 

3 

8 76 

4-5 

•1926 

9 

8-5 

8 

7-5 

7 

•0265 

2-0 

8 0 

4 

430 0 

6-0 

0-1838 

140-0 

210-0 

280 

350-0 

420 

•0254 

2-5 

8-75 

5 

1-25 

7-5 

•1751 

1 

1-5 

2 

2-5 

8 

•0244 

S-0 

9-5 

6 

2-5 

9-0 

•1670 

2 

3-0 

4 

5-0 

6 

•0233 

8-5 

260-25 

7 

3-75 

520-5 

•159 

3 

4-5 

6 

7-5 

9 

•0223 

4-0 

1-0 

8 

5-0 

2-0 

•1512 

4 

6-0 

8 

360-0 

432 

•0214 

4-5 

1-75 

9 

6-25 

3-5 

•1439 

5 

7-5 

290 

2-5 

5 

•0204 

5-0 

2-5 

350 

7-5 

6-0 

•1370 

6 

9-0 

2 

6-0 

8 

0-0195 

6-5 

3-25 

1 

8-75 

6-5 

•1300 

7 

220-5 

4 

7-5 

441 

-0186 

6-0 

4-0 

2 

440-0 

8-0 

•1235 

8 

2-0 

6 

370-0 

4 

•0177 

6-5 

4-75 

8 

1-25 

9-6 

•1172 

9 

3-6 

8 

2-5 

7 

•0169 

7-0 

5-5 

4 

2-5 

531 0 

O-llll 

150-0 

6-0 

800 

6-0 

450 

•0161 

7-5 

6-25 

5 

3-75 

2-6 

•1081 

0-5 

5-75 

1 

6-25 

1-5 

•0153 

8-0 

7-0 

6 

5-0 

4-0 

•1053 

1-0 

6-5 

2 

7-5 

3-0 

•0145 

8-5 

7-76 

7 

6-25 

5-5 

•1025 

1-6 

7-25 

8 

8-75 

4-5 

•0138 

9-0 

8-5 

8 

7-5 

7-0 

•0982 

2-0 

8-0 

4 

380-0 

6-0 

•0130 

9-5 

9-25 

9 

8-75 

8-6 

•0971 

2-5 

8-75 

5 

1-25 

7-5 

00124 

180-0 

270-0 

360 

450-0 

640-0 

•0944 

3-0 

9-5 

6 

2-5 

9-0 










































Table op Sizes, Weights and Eesistances for some important High Kesistance Materials at 60° F. 


I 


ELEGTRIGAL ENGINEERING TESTING 053 
Table XXXI. 





c 

o 

hH 

1 Resist- 

ranee bare. 

per lb. 

• 

Q • • • • • 

□ • • • • • 

A . 

o 

t- CO IC CO 
to O rH I-H 
. CO 04 04 

• O rH rH 

04 0 to 00 

CO to 00 CO 

b* p TJ< p p 

CQ b b b 
rH 04 CQ 

55-525 

92-588 

135-558 

206-613 

830-950 

554-592 

1045-978 

1490-08 

2678-47 

5601-09 

>• 

p p P 

b b b • • : 

0 CD • • • 

04 04 rH 

0 OS tO 
rH rH 00 

<i5 

0 

O 

CO 

O 

(V 

a 

Resist¬ 
ance. 
- 1 

per 1000 
yds. 

Ij-t O o 

a i <N 6 i 

5 O CO GO O (N 

'o 

O «p O <p 

b b* b b 

CO rH o CQ 
i-H 04 04 CQ 

p ^ p p p 

b b- b C4 b 

0 CO CO 0 CO 

CO 00 0 CQ b« 

rH rH rH 

0 p p p rH 

CQ b b b b 

0 CD CD to 

Tf 00 04 rH 

04 04 CQ ^ to 

to 
p p 

b» rH • J J 

CQ 0 

CD OS 


1 Manganin. 

Resistance. 

per 1000 
yds. 

03 

• • • • • 

^ • • • • • 

.d 

o 

00 74 

•U *22 • 

• rH rH • 

b 

p p 

r- b b 

04 :00 : CO 

CQ 140 t os 

1560-25 

2330-0 

to to to to p 
b b- b b 0 

^ 0 CD rH CD 

CS OS 0 

CQ CD 00 CQ 

rH 

20955-0 

32875-0 

47150-0 

73675-0 

• •• 

per lb. 

ohms. 

t-- 04 

04 to 

p p 

;b t-^ : 

15-652 

49-475 

135-175 

354-700 

791-475 

1731-750 

3514-25 

6107-25 

11597-75 

24904-50 

64100-0 

156525-0 

324550-0 

792375-0 

• •• 

• • • 

Eureka. 

Resistance. 

per 1000 
yds. 

. Tjl to 

OQ • • 

g lO , O .CO 
□ CO to t CO 

o 

<p p 

rH rH 

• • 

• fH • 04 • 

rH CQ to t- 

b- 0 CO S 
cs CO 0 00 ^ 
CQ to 00 

CD tO OS 04 to 
b- b- CD 0 
to 00 04 00 CQ 
rH rH 04 04 CQ 

'^•^CQtOb- 
0 00 04 rH 
rH CS b» b^ b» 

lOb^ 0 to 

rH rH 

CQ 0 0 0 

rH 0 to 0 »0 

04 b- CD CD CS 

to OS CD 00 b* CD 

C4 CQ to 00 to 04 
rH 04 

per lb. 

ohms. 

•152 

• •• 

•372 

••• 

•855 

04 CQ 

CO 

p 

:b *b t 

• • • 

00000 

rf b- UQ CQ 0 
p p p ^ 

00 OS OS 0 04 
rH CQ tO OS CD 
rH 

301-500 

427-000 

625-200 

952-900 

1382 

tO rH 04 to 

00 CQ to CD 00 
0 04 CQ OS CS 
04 |> CQ OS 

rH 04 

77187 

188440 

890700 

903880 

3015100 

6252300 

Platinoid. 

Resistance bare 
(approx.) 

per 1000 
yds. 

04 <N 

. O 00 Ci 

CQ 00 O 04 

^ • • • • • 

c 00 : o ; 00 

^04 to 

O 

CO 00 

rH CQ 

: 

•to *0 • 

rH 00 

rH rH 

320-601 

461-664 

569-952 

721-368 

942-192 

1282-392 

1526-184 

1846-656 

2279-808 

2746-440 

3372-264 

4803-984 

6332-904 

8727-120 

12789-640 

20518-560 

32060-160 

46166 

72136 

128239 

184665 

per lb. 

ohms. 

•1241 

‘•3031 

•6957 

V 04 0 

CO 04 

00 to 

too : 

•rH • 

15-3312 

31-7952 

48-4602 

77-6480 

132-4272 

245-3280 

347-4720 

508-7280 

775-3680 

1125-2160 

1696-4880 

8442-8000 

5982-7200 

11362 

24398 

62805 

153333 

817904 

784280 

2453280 

5087280 

German Silver. 

Resistance bare 
(approx.) 

per 1000 

yds. 

Ci iC O O 04 

• 04 CO Oi O O 

S 2 Oi CO CO cc 1 — 

B * * * * * 

i-i I-I 04 CO CQ 

o 

Oi Ci 04 rH CQ 
rH CO CO CQ 
hH b- CO 0 p 

b b b b b 

CO 04 CQ 

rH 

OS b- CD tO to 
p p p p 74 

b b b 

b- to rH os 04 
rH C4 CQ CQ 0 

707-98 

843-58 

1019-50 

1258-60 

1516-20 

74 p 74 p 

Ah b b b b 

to tO os f-H CO 
00 CD 00 0 
rH 04 CQ b- 

b^ OS »o 00 0 

04 OS op 04 OS »D 
CQ CD 00 b- OS 

rH b* to Os rH 

rH rH 04 CQ b* 0 
rH 

per lb. 

00 00 

CO o CO 00 

2 O r-« r-l 04 CQ 

B • • • • • 

1 

o 

b« 0 0 0 0 

04 0 b- b- CO 
p p p p p 
A 4 04 b 

00000 

CD tC 0 tO rH 
p ^ p 7 H 

00 b- CD b b 
rH CH b- 

P p rH P 
to rH 0 00 rH 
CQ Os 00 C4 C4 
rH rH ©4 CD 

0 M <N 3S 
CO 0 0 0 

05 Oi CO <N 

ft eo <0 CO 

rH 

34673 

84652 

175509 

428571 

1354415 

2808602 

Copper. 

Resistance 
bare (approx). 

pr 1000 

yds. 

. 00 O 04 04 00 

W UQ 00 

^ iH Ah iH ^ (N 
O 

OS b* 0 

CO rH rH to 04 
p ^ p p p 

b b b b- b 

1 :^ b-. 0 0 0 

OS to 00 OS CO 
p p p 

b 00 b b 

rH rH <N 04 CQ 

00000 

OS rH 00 0 0 
p p p 

b b b b b 

tD CD Os rH 

rH 

p ^ p 

b* b b b b 

CQ os to to 04 
rH rH 04 P tO 

04 to CD b^ O* CO 
X rH CO -rji CQ 

S CQ 00 OS 04 to 
g rH rH 04 to b- 

per. lb. 

, l-H t- CQ 

00 lO 04 00 CO 

G O O r-t I-* 04 

C o O O O O 

.. 

o 

Tt< rH b« rH OS 
COrHCQOOb- 
GO 04 OS CQ 
0 0 rH rH CQ 

rHOOOSOSb- 
CD OS b- CD 0 
04 04 OS rH 

co • • • • 

V rH rH CQ 0 

00 C4 OS CQ 
rH 00 to ri< 

0 7 ^ ^ p p 
b b A-< b 

rH rH C4 CQ 

69-270 

140-670 

244-290 

463-910 

996-180 

rH 04 to rH 0 
CD CD 00 OS b- 04 
to 04 OS CD rH b- 
04COC4rHOb. 
rH CQ 0 0 
r-i G4 

Yds. to the lb. 

Covered 

(approx.) 

Cottn 

covd. 

lO 04 CO O 04 

^ O ^ ^ ^ 

4t« lb A- 6 > 

00000 

p b- p p 

04 b b 04 
rH fH rH C4 CQ 

00000 

0 0 0 CQ 
• • • • • 

to cs 0 0 OS 
CD 00 0 04 

rH rH 

00000 

p p ^ 00 

b to b b b 

rH CD rH b» 

rH 04 ©4 CQ CQ 

0 0 0 CQ CQ 

C4 rH 04 CD rH 
^ ^ .X »-H CD 

b* Mrf 

CDb-b- 
CD 00 

• •••«« 

Silk 1 

covd.l 

to 00 CO CO 04 

^ Ip o o 
b b b b 

rH 

0 0 b^ 0 0 

OS p p 04 

04 b b b 

rH rH 04 04 CQ 

00000 

p p p P 
b» rH CQ tO CO 
b- 00 0 CQ 

rH rH 

00000 

p p p ^ p 
b ^ 04 b b 
00 C4 CQ OS 

rH 01 04 CQ CQ 

0 0 0 OS rH 

00 rH 04 CQ OS 

CS OS rH 

CD OS 

CD to CQ 04 0 

0 CD OS rH 0 

COCQb-04b«b- 
04 Hj« tO 00 CQ 0 
rH 04 

Bare. 

« M ^ W 04 

b b b b b 

rH 

p p p p 74 

b b- CO to 

rH rH 04 04 CQ 

p p p p 

b b b b b 

CD 00 0 

rH rH 

04 CO to (-H 00 

rH b^ tO 0 OS 
OS 04 0 

rH 04 04 CQ 

CO CQ CQ rH CD 

• • • r-t 

0 rH Tj< ^ 

tC b«* OS 

CS 0 C4 tO 0 00 

to 0 00 tO 04 -rt< 
0 00 QO b- rH lO 
CQ ^ CD 0 Os 

rH r- C4 


Pi OOOT 
d 

O 00 C4 04 
^ ^ 

,Q C4 b b 04 b 
' CO 00 04 OJ 

04 rH 

CD 04 00 OS CO 

p 7 H p p 

b b b- A- b 

b« to TtH CQ 04 

04 CO b- OS 04 
OS to b- 04 rH 
• • • • • 

0 rH OS b» 

04 rH rH 

CQ OS CQ ^ 

p p p p 

b b b b 

OS 0 CD b» 04 
OS 0 b» 0 

• • • • • 
rH rH rH 

tO CQ 04 CD 

CQ rH OS to CQ 

CQ 04 rH 0 0 0 

• ••••* 

Size. 1 

a 

'a 

00 rH CO 04 
to lO ^ ^ 

o CO 04 04 CO 

tH CQ b 04 04 

CO 04 OS CO 04 
CQ CQ 04 04 04 

p p p p 

04 04 rH rH rH 

OS CD CQ rH 

1 —* ^ rH rH rH 

P p p p ^ 

rH rH 

OOSCOt^b* 

rH to 0 ‘D rH 

CD tO 0 ^ ^ 

• • • • • 

0 

CD tO -H’ CQ CQ 
b^ rH b- CQ OS 
CQ CQ 04 04 rH 

• • • • • 

Os CD CQ 0 CO 

04 rH rH rH rH O 
to 04 0 00 CD uQ 
TH 7 H 7 H p p p 

Inch. 

O r#i 00 CO 

CO Tt< 04 rH O 
rH rH rH iH rH 

04 0 04 -rjl CO 
OS CO b- CO tO 

00000 

00 0 <0 (N 00 

05 eo (N 

00000 

^ 04 0 00 CD 
04 04 04 rH rH 

00000 

00't** CO 04 CD 

04 0 OS b- 

rH rH p-H 0 0 

00000 

0 00 0 04 0 

CO -Hi CQ 04 04 

000000 
0 00000 

L.S.Q. 

OOOkOrHCI 
rH rH rH 

CQ ^ tO CO b* 
rH ^ rH rH rH 

00 OS 0 rH 04 
rH rH 04 04 04 

CQ ^ p COb* 
04 ^ ^ C4 04 

00 0 04 CD 

04 CQ OQ CQ CQ 

oooet-<i<«ot~ 
00 •« '<«< 



Resistance of Copper calculated on a basis of 100% of pure Copper at 60* P. German Silver calculated on F. Jenkins’ Table, 






















































































































654 


ELEGTIUGAL ENGINEERING TESTING 


Table XXXII. 

The Table given below shows the sizes op Various Wires op 
Different Materials which will fuse at the Currents 
GIVEN IN THE FIRST COLUMN (SiR W. H. PrEECB). 


Current 

in 

Amjieres. 

Tin Wire. 

Lead Wire. 

Copper Wire. 

Iron Wire. 

Diameter 

Inches. 

Approx. 

S.W.G. 

Diameter 

Inches. 

Approx. 

S.W.G. 

Diameter 

Inches. 

Approx. 

S.W.G. 

Diameter 

Inches. 

Approx. 

S.W.G. 

1 

0-0072 

36 

0-0081 

35 

0 0021 

47 

0 0047 

40 

2 

0-0113 

81 

00128 

80 

0-0034 

43 

0-0074 

36 

8 

0-0149 

28 

0 0168 

27 

0-0044 

41 

0 0097 

33 

4 

0-0181 

26 

0 0203 

25 

0-0053 

39 

0-0117 

31 

5 

0-0210 

25 

0 0236 

23 

0-0062 

38 

0-0136 

29 

10 

0-0334 

21 

0-0375 

20 

0 0098 

83 

0-0216 

24 

15 

0-0437 

19 

0 0491 

18 

0-0129 

80 

0 0283 

22 

20 

0-0529 

17 

0-0595 

17 

0-0156 

28 

0 0343 

20-5 

25 

0-0614 

16 

0-0690 

15 

00181 

26 

0-0398 

19 

30 

0-0694 

15 

0-0779 

14 

0-0205 

25 

0 0450 

18-5 

35 

0-0769 

14-5 

0-0864 

13-5 

0-0227 

24 

0-0498 

18 

40 

0-0840 

13-5 

0-0944 

13 

0-0248 

23 

0-0545 

17 

45 

0-0909 

13 

0-1021 

12 

0-0268 

22 

0-0589 

16-5 

50 

0-0975 

12-5 

0-1095 

11-5 

0-0288 

22 

0 0632 

16 

60 

0-1101 

11 

0-1237 

10 

0-0325 

21 

0-0714 

15 

70 

0-1220 

10 

0-1371 

9-5 

0-0360 

20 

0-0791 

14 

80 

0-1334 

9-5 

0-1499 

8-5 

0-0394 

19 

0-0864 

13-5 

90 

0-1443 

9 

0-1621 

8 

0-0426 

18-5 

0-0935 

13 

100 

0-1548 

8-5 

0-1739 

7 

0-0457 

18 

0-1003 

12 

120 

0-1748 

7 

0-1964 

6 

0-0516 

17-5 

0-1133 

11 

140 

0-1937 

6 

0-2176 

5 

0-0572 

17 

0-1255 

10 

160 

0-2118 

6 

0-2379 

4 

0-0625 

16 

0-1372 

9-5 

180 

0-2291 

4 

0-2573 

3 

0-0676 

16 

0-1484 

9 

200 

0-2457 

3-5 

0-2760 

2 

0-0725 

15 

0-1592 

8 

250 

0-2851 

1-5 

0 3203 

0 

0-0841 

13-5 

0-1848 

6-5 


Note.— ^Tlie above numbers can only be talcen as approximate, as the actual current 
required to fuse any gauge will depend on the length of fuse and cooling effects of the fuse 
block in which it is placed. 

In Allo-tin diameters 3*0 % greater than those of lead fuse at the same currents. 


Useful Numbers. 


Metres 

X 

3-2809 

= 

feet. 

Feet 

X 

0-3048 


metres. 

Centimetres 

X 

0-3937 

= 

inches. 

Inches 

X 

2-54 

— 

cms. 

Mils. 

X 

0-0254 

= 

millimetres. 

Sq. cms. 

X 

0-155 

= 

sq. inches. 

Sq. inches 

X 

000155 


sq. mm. 

Sq. inches 

X 

6-451 

= 

sq. cms. 

Cubic „ 

X 

16-387 


cub. „ 

„ cms. 

X 

0-061027 

=: 

,, inches. 



























ELECTRICAL ENGINEERING TESTING 


655 


Kilogrammes X 

2*2046 


= pounds. 

Miles X 

1*609 


= kilometres. 

Kilometres x 

1094 


= yards. 

Pounds (avoir.) x 

0*4536 

= 

kilogrammes. 

Gallons 

0*1604 

= 

cubic ft. 

Cubic ft. 

6*2355 

= 

gallons. 

Gallons (water) x 

10*0 

= 

pounds. 

Cub. ft. ( „ ) X 

62*27 

= 

>5 

Metres x 

39*3704 

= 

inches. 

Foot X 

30*4797 

= 

cms. 

Pounds (avoir.) x 

453*593 

= 

grammes. 

Eevs. per sec. x 

6*2832 

= 

radians per sec, 

Feet ,, min, x 

0*005 

= 

metres per „ 

Metres,, sec. x 

197 

= 

ft. per min. 


Joules Equivalent = 1390 lb. cent, units 

= 4‘156 X 10'^ ergs per gram. °C. 
Acceleration due to Gravity (g) at Greenwich 

= 32*1908 ft. sec. units 
= 981*17 cm. „ „ 

Density of mercury = 13 596 grammes per c.c. 


Watts 

X 

0*7373 

= 

foot lbs. per sec. 

Joules {i. e. Watt secs.) 

X 

107 

= 

ergs. 

» » 

X 

0*7373 

= 

ft. lbs. 

)> »♦ 

X 

0*239 

= 

calorics. 

Calorics 

X 

4*158 

= 

Joules. 

Ft. lbs. 

X 

1*35 

= 

ff 

Kilogrammeters 

X 

7*233 

=: 

ft. lbs. 

Ft. lbs. 

X 

0*138 

= 

kilogrammeters. 

Horse-power 

X 

746 

= 

Watts. 

tf n 

X 

33000 

= 

ft. lbs. per min. 

?> »> 

X 

550 

= 

if ff sec. 


X 

76 

= 

kilogrammeters per sec. 

Watts 

X 

44*25 

= 

ft. lbs. per min. 

if 

X 

0 1 

= 

kilogrammeters per sec. 

H.P. hours 

X 

1980000 

= 

ft. lbs. 

if a 

X 

2685600 

= 

Joules. 

Kilowatt hours 

X 

1*34 

= 

H.P. hours. 

if ft a 

X 

2656400 

= 

ft. lbs. 


Length of circumference of circle radius (r) = 27rt’ = 7rd. 


650 


ELEGTRIOAL ENGINEERING TESTING 


Trd^ 


Area of circumference of circle radius (r) = = *7854(^2, 


Eatio of circumference of circle to its diam. 

(tt) = 3*14159 = V' approx, 
log. 7r = 0*4971499. 

Base of hyperbolic or Napierian logarithms € = 2*71828 . . . 
To convert Napierian into Common logarithms multiply by 
0*43429. 

To convert Common into Napierian logarithms multiply by 
2*30258. 

Lbs. per yard of pure copper wire = area in sq. ins. x 11*5625. 
Ohms per yard of pure copper wire at 60° F. (15'5°C.) 

= 0-0000244657 4-area in sq. ins. 

Pounds per 1000 yards -)- 10 % = pounds per kilometre. 

„ „ 1000 „ -f- 2 = kilograms per „ 


ELEGTRIOAL ENGINEERim TESTING 


057 


Logarithms. 



0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

1 2 

3 4 

5 

6 7 

8 9 

10 

)000 

)043 

3086 

0128 

0170 

0212 

0253 

0294 

0334 

0374 

4 8 

12 17 

21 

25 29 

33 37 

11 

0414 

0453 

0492 

0531 

0569 

0607 

0645 

0682 

0719 

0755 

4 8 

11 15 

19 

23 26 

30 34 

12 

0792 

0820 

0804 

0899 

0934 

0969 

1004 

1038 

1072 

1106 

3 7 

10 14 

17 

21 24 

28 31 

13 

1139 

1173 

1206 

1239 

1271 

1303 

1335 

1367 

1399 

1430 

3 6 

10 13 

16 

19 23 

26 29 

14 

1401 

1492 

1523 

1553 

1584 

1614 

1644 

1673 

1703 

1732 

3 6 

9 12 

15 

18 21 

24 27 

15 

1761 

1790 

1818 

1847 

1875 

1903 

1931 

1959 

1987 

2014 

3 6 

8 11 

14 

17 20 

22 25 

16 

2041 

2068 

2095 

2122 

2148 

2175 

2201 

2227 

2253 

2279 

3 5 

8 11 

13 

16 18 

21 24 

17 

2304 

2330 

2355 

2380 

2405 

2430 

2455 

2480 

2504 

2529 

2 5 

7 10 

12 

15 17 

20 22 

18 

2553 

2577 

2601 

2625 

2648 

2672 

2695 

2718 

2742 

2765 

2 5 

7 9 

12 

14 16 

19 21 

19 

2788 

2810 

2833 

2856 

2878 

2900 

2923 

2945 

2967 

2989 

2 4 

7 9 

11 

13 16 

18 20 

20 

3010 

3032 

3054 

3075 

3096 

3118 

3139 

3160 

3181 

3201 

2 4 

6 8 

11 

13 15 

17 19 

21 

3222 

3243 

3263 

3284 

3304 

3324 

3345 

3365 

3385 

3404 

2 4 

6 8 

10 

12 14 

16 18 

22 

3424 

3444 

3464 

3483 

3502 

3522 

3541 

3560 

3579 

3598 

2 4 

6 8 

10 

12 14 

15 17 

23 

3617 

3636 

3655 

3674 

3692 

3711 

3729 

3747 

3766 

3784 

2 4 

6 7 

9 

11 13 

15 17 

24 

3802 

3820 

3838 

3856 

3874 

3892 

3909 

3927 

3945 

3962 

2 4 

5 7 

9 

11 12 

14 16 

25 

3979 

3997 

4014 

4031 

4048 

4065 

4082 

4099 

4116 

4133 

2 3 

5 7 

9 

10 12 

14 15 

26 

4150 

4166 

4183 

4200 

4216 

4232 

4249 

4265 

4281 

4298 

2 3 

5 7 

8 

10 11 

13 15 

27 

4314 

4330 

4346 

4362 

4378 

4393 

4409 

4425 

4440 

4456 

2 3 

5 6 

8 

9 11 

13 14 

28 

4472 

4487 

4502 

4518 

4533 

4548 

4564 

4579 

4594 

4609 

2 3 

5 6 

8 

9 11 

12 14 

29 

4624 

4639 

4654 

4669 

4683 

4698 

4713 

4728 

4742 

4757 

1 3 

4 6 

7 

9 10 

12 13 

30 

4771 

4786 

4800 

4814 

4829 

4843 

4857 

4871 

4886 

4900 

1 3 

4 6 

7 

9 10 

11 13 

31 

4914 

4928 

4942 

4955 

4969 

4983 

4997 

5011 

5024 

5038 

1 3 

4 6 

7 

8 10 

11 12 

|32 

5051 

5065 

5079 

5092 

5105 

5119 

5132 

5145 

5159 

5172 

1 3 

4 5 

7 

8 9 

11 12 

|33 

5185 

5198 

5211 

5224 

5237 

5250 

5263 

5276 

5289 

5302 

1 3 

4 5 

6 

8 9 

10 12 

|34 

5315 

5328 

5340 

5353 

5366 

5378 

5391 

5403 

5416 

5428 

1 3 

4 5 

6 

8 9 

10 11 

35 

5441 

5453 

5465 

5478 

5490 

5502 

5514 

5527 

5539 

5551 

1 2 

4 5 

6 

7 9 

10 11 

36 

5563 

5575 

5587 

5599 

5611 

5623 

5635 

5647 

5658 

5670 

1 2 

4 5 

6 

7 8 

10 11 

37 

5682 

5694 

5705 

5717 

5729 

5740 

5752 

5763 

5775 

5786 

1 2 

3 5 

6 

7 8 

9 10 

38 

5793 

5809 

5821 

5832 

5843 

5855 

5866 

5877 

5888 

5899 

1 2 

'6 5 

6 

7 8 

0 10 

39 

5911 

5922 

5933 

5944 

5955 

5966 

5977 

5988 

5999 

6010 

1 2 

3 4 

5 

7 8 

9 10 

40 

6021 

6031 

6042 

6053 

6064 

6075 

6085 

6096 

6107 

6117 

1 2 

3 4 

5 

6 8 

9 10 

41 

6128 

6138 

6149 

6160 

6170 

6180 

6191 

6201 

6212 

6222 

1 2 

3 4 

5 

6 7 

8 9 

42 

6232 

6243 

6253 

6263 

6274 

6284 

6294 

6304 

6314 

6325 

1 2 

3 4 

5 

6 7 

0 9 

43 

6335 

6345 

6355 

6365 

6375 

6385 

6395 

6405 

6415 

6425 

1 2 

3 4 

5 

6 7 

o 9 

44 

6435 

6444 

6454 

6464 

6474 

6484 

6493 

6503 

6513 

6522 

1 2 

3 4 

0 

6 7 

o 9 

45 

6532 

6542 

6551 

6561 

6571 

6580 

6590 

6599 

6609 

6618 

1 2 

3 4 

5 

6 7 

o 9 

46 

6628 

6637 

6646 

6656 

6665 

6675 

6684 

6693 

6702 

6712 

1 2 

3 4 

5 

6 7 

7 8 

*7 K 

47 

6721 

6730 

6739 

6749 

6758 

6767 

6776 

6785 

6794 

6803 

1 2 

3 4 

5 

5 D 

( o 

48 

6812 

6821 

6830 

6839 

6848 

6857 

6866 

6875 

6884 

6893 

1 2 

3 4 

4 

5 0 

i o 

49 

6902 

6911 

6920 

6928 

6937 

6946 

6955 

6964 

6972 

6981 

1 2 

3 4 

4 

5 D 

1 o 

*7 R 

50 

6990 

6998 

7007 

7016 

7024 

7033 

7042 

7050 

7059 

7067 

1 2 

3 3 

4 

5 o 

1 o 

51 

7076 

7084 

7093 

7101 

7116 

7118 

7126 

7135 

7143 

7152 

1 2 

3 3 

4 

5 6 

7 8 
7 7 

52 

716C 

7168 

7177 

7186 

7193 

7202 

7216 

7218 

7226 

7235 

1 2 

2 3 

4 

0 0 

7 

53 

724J 

7251 

725S 

7267 

7275 

7284 

7292 

7300 

7308 

7316 

1 2 

2 3 

4 

0 0 

V 1 

6 7 

54 

7324 

7332 

734C 

734J 

7356 

7364 

7372 

7380 

7388 

7396 

1 2 

2 3 

4 

0 0 











































































658 


ELECTBIGAL ENGINEERING TESTING 


Logarithms. 



0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

1 2 

8 4 

5 

6 

7 

8 

9 

65 

7404 

7412 

7419 

7427 

7435 

7443 

7451 

7459 

7466 

7474 

1 

2 

2 

3 

4 

5 

6 

6 

7 

56 

7482 

7490 

7497 

7505 

7513 

7520 

7528 

7536 

7543 

7551 

1 

2 

2 

3 

4 

5 

5 

6 

7 

67 

7559 

7566 

7574 

7682 

7589 

7597 

7604 

7612 

7619 

7627 

1 

2 

2 

3 

4 

5 

5 

6 

7 

68 

7634 

7642 

7649 

7657 

7664 

7672 

7679 

7686 

7694 

7701 

1 

1 

2 

3 

4 

4 

5 

6 

7 

59 

7709 

7716 

7723 

7731 

7738 

7745 

7752 

7760 

7767 

7774 

1 

1 

2 

3 

4 

4 

5 

6 

• 

60 

7782 

7789 

7796 

7803 

7810 

7818 

7825 

7832 

7859 

7846 

1 

1 

2 

3 

4 

4 

5 

6 

6 

61 

7853 

7860 

7868 

7875 

7882 

7889 

7896 

7903 

7910 

7917 

1 

1 

2 

3 

4 

4 

5 

6 

6 

62 

7924 

7931 

7938 

7945 

7952 

7959 

7966 

7973 

7980 

7987 

1 

1 

2 

3 

3 

4 

5 

6 

6 

63 

7993 

8000 

8007 

8014 

8021 

8028 

8035 

8041 

8048 

8055 

1 

1 

2 

3 

3 

4 

5 

5 

6 

64 

8062 

8069 

8075 

8082 

8089 

8096 

8102 

8109 

8116 

8122 

1 

1 

2 

3 

3 

4 

5 

5 

6 

65 

8129 

8136 

8142 

8149 

8156 

8162 

8169 

8170 

8182 

8189 

1 

1 

2 

3 

3 

4 

5 

5 

6 

66 

8195 

8202 

8209 

8215 

8222 

8228 

8235 

8241 

8248 

8254 

1 

1 

2 

3 

3 

4 

5 

5 

6 

67 

8261 

8267 

8274 

8280 

8287 

8293 

8299 

8306 

8312 

8319 

1 

1 

2 

8 

3 

4 

6 

5 

6 

68 

8325 

8331 

8338 

8344 

8351 

8357 

8363 

8370 

8376 

8382 

1 

1 

2 

8 

3 

4 

4 

5 

6 

69 

8388 

8395 

8401 

8407 

8414 

8420 

8426 

8432 

8439 

8445 

1 

1 

2 

2 

3 

4 

4 

6 

6 

70 

8451 

8457 

8463 

8470 

8476 

8482 

8488 

8494 

8500 

8506 

1 

1 

2 

2 

8 

4 

4 

5 

6 

71 

8513 

8519 

8525 

8531 

8537 

8543 

8549 

8555 

8561 

8567 

1 

1 

2 

2 

3 

4 

4 

5 

5 

72 

8573 

8579 

8585 

8591 

8597 

8603 

8609 

8615 

8621 

8627 

1 

1 

2 

2 

3 

4 

4 

5 

b 

73 

8633 

8639 

8645 

8651 

8657 

8663 

8669 

8675 

8681 

8686 

1 

1 

2 

2 

3 

4 

4 

5 

6 

74 

8692 

8698 

8704 

8710 

8716 

8722 

8727 

8733 

8739 

8745 

1 

1 

2 

2 

3 

4 

4 

5 

6 

76 

8751 

8756 

8762 

8768 

8774 

8779 

8785 

8791 

8797 

8802 

1 

1 

2 

2 

3 

3 

4 

5 

6 

76 

8808 

8814 

8820 

8825 

8831 

8837 

8842 

8848 

8854 

8859 

1 

1 

2 

2 

3 

3 

4 

5 

6 

77 

8865 

8871 

8876 

8882 

8887 

8893 

8899 

8904 

8910 

8915 

1 

1 

2 

2 

3 

3 

4 

4 

5 

78 

8921 

8927 

8932 

8938 

8943 

8949 

8954 

8960 

8965 

8971 

1 

1 

2 

2 

3 

3 

4 

4 

5 

79 

8976 

8982 

8987 

8993 

8998 

9004 

9009 

9015 

9020 

9025 

1 

1 

2 

2 

3 

3 

4 

4 

5 

80 

9031 

9036 

9042 

9047 

9053 

9058 

9063 

9069 

9074 

9079 

1 

1 

2 

2 

3 

3 

4 

4 

5 

81 

9085 

9090 

9096 

9101 

9106 

9112 

9117 

9122 

9128 

9133 

1 

1 

2 

2 

8 

3 

4 

4 

5 

82 

9138 

9143 

9149 

9154 

9159 

9165 

9170 

9175 

9180 

9186 

1 

1 

2 

2 

3 

3 

4 

4 

5 

83 

9191 

9196 

9201 

9206 

9212 

9217 

9222 

9227 

9232 

9238 

1 

1 

2 

2 

3 

3 

4 

4 

5 

84 

9243 

9248 

9253 

9258 

9263 

9269 

9274 

9279 

9284 

9289 

1 

1 

2 

2 

3 

3 

4 

4 

6 

85 

9294 

9299 

9304 

9309 

9315 

9320 

9325 

9330 

9335 

9340 

1 

1 

2 

2 

3 

3 

4 

4 

6 

86 

9345 

9350 

9355 

9360 

9365 

9370 

9375 

9380 

9385 

9390 

1 

1 

2 

2 

3 

8 

4 

4 

5 

87 

9395 

9400 

9405 

9410 

9415 

9420 

9425 

0430 

9435 

9440 

0 

1 

1 

2 

2 

8 

3 

4 

4 

88 

9445 

9450 

9456 

9460 

9465 

9469 

9474 

9479 

9484 

9489 

0 

1 

1 

2 

2 

3 

3 

4 

4 

89 

9494 

9499 

9504 

9509 

9513 

9518 

9523 

9528 

9533 

9538 

0 

1 

1 

2 

2 

3 

8 

4 

4 

90 

9542 

9547 

9552 

9657 

9562 

9566 

9571 

9576 

9581 

9586 

0 

1 

1 

2 

2 

8 

8 

4 

4 

91 

9590 

9595 

9600 

9605 

9609 

9614 

9619 

9624 

9628 

9633 

0 

1 

1 

2 

2 

3 

8 

4 

4 

92 

9638 

9643 

9647 

9652 

9657 

9661 

9666 

9671 

9675 

9680 

0 

1 

1 

2 

2- 

3 

3 

4 

4 

93 

9685 

9689 

9694 

9699 

9703 

9708 

9713 

9717 

9722 

9727 

0 

1 

1 

2 

2 

3 

8 

4 

4 

94 

9731 

9736 

9741 

9745 

9750 

9754 

9759 

9763 

9768 

9773 

0 

1 

1 

2 

2 

3 

3 

4 

4 

95 

9777 

9782 

9786 

9791 

9795 

9800 

9805 

9809 

9814 

9818 

0 

1 

1 

2 

2 

3 

3 

4 

4 

96 

9823 

9827 

9832 

9836 

9841 

9845 

9850 

9854 

9859 

9863 

0 

1 

1 

2 

2 

3 

8 

4 

4 

97 

9868 

9872 

9877 

9881 

9886 

9890 

9894 

9899 

9903 

9908 

0 

1 

1 

2 

2 

3 

8 

4 

4 

98 

9912 

9917 

9921 

9926 

9930 

9934 

9939 

9943 

9948 

9952 

0 

1 

1 

2 

2 

3 

3 

4 

4 

99 

9956 

9961 

9965 

9969 

9974 

9978 

9983 

9987 

9991 

9996 

0 

1 

1 

2 

2 

3 

3 

3 

4 







































ELECTRICAL ENGINEERING TESTING 


659 


Anti-logarithms. 



0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

1 

2 

3 

4 

5 

6 

7 

8 

9 

•00 

1000 

1002 

1005 

1007 

1009 

1012 

1014 

1016 

1019 

1021 

0 

0 

1 

1 

1 

1 

2 

2 

2 

•01 

1023 

1026 

1028 

1030 

1033 

1035 

1038 

1040 

1042 

1045 

0 

0 

1 

1 

1 

1 

0 

2 

9 

•02 

1047 

1050 

1052 

1054 

1057 

1059 

1062 

1064 

1067 

1069 

0 

0 

1 

1 

1 

1 

2 

2 

9 

•03 

1072 

1074 

1076 

1079 

1081 

1084 

1086 

1089 

1091 

1094 

0 

0 

1 

1 

1 

1 

2 

2 

2 

•04 

1096 

1099 

1102 

1104 

1107 

1109 

1112 

1114 

1117 

1119 

0 

1 

1 

1 

1 

2 

2 

2 

2 

•05 

1122 

1125 

1127 

1130 

1132 

1135 

1138 

1140 

1143 

1146 

0 

1 

1 

1 

1 

2 

2 

2 

2 

•06 

1148 

1151 

1153 

1156 

1159 

1161 

1164 

1167 

1169 

1172 

0 

1 

1 

1 

1 

2 

2 

2 

2 

•07 

11(5 

1178 

1180 

1183 

1186 

1189 

1191 

1194 

1197 

1199 

0 

1 

1 

1 

1 

2 

2 

2 

2 

08 

1202 

1205 

1208 

1211 

1213 

1216 

1219 

1222 

1225 

1227 

0 

1 

1 

1 

1 

2 

2 

2 

3 

•09 

1230 

1233 

1236 

1239 

1242 

1245 

1247 

1250 

1253 

1256 

0 

1 

1 

1 

1 

2 

2 

2 

3 

•10 

1259 

1262 

1265 

1268 

1271 

1274 

1276 

1279 

1282 

1285 

0 

1 

1 

1 

1 

2 

2 

2 

8 

•11 

1288 

1291 

1294 

1297 

1300 

1303 

1306 

1809 

1312 

1315 

0 

1 

1 

1 

2 

2 

2 

2 

3 

•12 

1318 

1321 

1324 

1827 

1330 

1334 

1337 

1340 

1343 

1346 

0 

1 

1 

1 

2 

2 

2 

2 

3 

•13 

1349 

1352 

1355 

1358 

1361 

1365 

1368 

1371 

1374 

1377 

0 

1 

1 

1 

2 

2 

2 

3 

3 

•14 

1380 

1384 

1387 

1390 

1393 

1396 

1400 

1403 

1406 

1409 

0 

1 

1 

1 

2 

2 

2 

3 

3 

•15 

1413 

1416 

1419 

1422 

1426 

1429 

1432 

1435 

1439 

1442 

0 

1 

1 

1 

2 

2 

2 

3 

3 

•16 

1445 

1449 

1452 

1455 

1459 

1462 

1466 

1469 

1472 

1476 

0 

1 

1 

1 

2 

2 

2 

3 

3 

•17 

1479 

1483 

1486 

1489 

1493 

1496 

1500 

1503 

1507 

1510 

0 

1 

1 

1 

2 

2 

2 

3 

3 

•18 

1514 

1517 

1521 

1524 

1528 

1531 

1535 

1538 

1542 

1545 

0 

1 

1 

1 

2 

2 

2 

3 

3 

•19 

1549 

1552 

1556 

1560 

1563 

1567 

1570 

1574 

1578 

1581 

0 

1 

1 

1 

2 

2 

3 

3 

3 

•20 

1585 

1589 

1592 

1596 

1600 

1603 

1607 

1611 

1614 

1618 

0 

1 

1 

1 

2 

2 

3 

3 

3 

•21 

1622 

1626 

1629 

1633 

1637 

1641 

1644 

1648 

1652 

1656 

0 

1 

1 

2 

2 

2 

3 

3 

3 

•22 

1660 

1663 

1667 

1671 

1675 

1679 

1683 

1687 

1690 

1694 

0 

1 

1 

2 

2 

2 

3 

3 

3 

•23 

1698 

1702 

1706 

1710 

1714 

1718 

1722 

1726 

1730 

1734 

0 

1 

1 

2 

2 

2 

3 

3 

4 

•24 

1738 

1742 

1746 

1750 

1754 

1758 

1762 

1766 

1770 

1774 

0 

1 

1 

2 

2 

2 

3 

3 

4 

*25 

1778 

1782 

1786 

1791 

1795 

1799 

1803 

1807 

1811 

1816 

0 

1 

1 

2 

2 

2 

3 

3 

4 

•26 

1820 

1824 

1828 

1832 

1837 

1841 

1845 

1849 

1854 

1858 

0 

1 

1 

2 

2 

3 

3 

3 

4 

•27 

1862 

1866 

1871 

1875 

1879 

1884 

1888 

1892 

1897 

1901 

0 

1 

1 

2 

2 

3 

3 

3 

4 

•28 

1905 

1910 

1914 

1919 

1923 

1928 

1932 

1936 

1941 

1945 

0 

1 

1 

2 

2 

3 

8 

4 

4 

•29 

1950 

1954 

1959 

1963 

1968 

1972 

1977 

1982 

1986 

1991 

0 

1 

1 

2 

2 

3 

8 

4 

4 

•30 

1995 

2000 

2004 

2009 

2014 

2018 

2023 

2028 

2032 

2037 

0 

1 

1 

2 

2 

3 

3 

4 

4 

•31 

2042 

2046 

2051 

2056 

2061 

2065 

2070 

2075 

2080 

2084 

0 

1 

1 

2 

2 

3 

3 

4 

4 

•32 

2089 

2094 

2099 

2104 

2109 

2113 

2118 

2123 

2128 

2133 

0 

1 

1 

2 

2 

3 

3 

4 

4 

•33 

2138 

2143 

2148 

2153 

2158 

2163 

2168 

2173 

2178 

2183 

0 

1 

1 

2 

2 

3 

3 

4 

4 

•34 

2188 

2193 

2198 

2203 

2208 

2213 

2218 

2223 

2228 

2234 

1 

1 

2 

2 

3 

3 

4 

4 

5 

•35 

2239 

2244 

2249 

2254 

2259 

2265 

2270 

2275 

2280 

2286 

1 

1 

2 

2 

3 

3 

4 

4 

5 

•36 

2291 

2296 

2301 

2307 

2312 

2317 

2323 

2328 

2333 

2339 

1 

1 

2 

2 

8 

8 

4 

4 

6 

•37 

2344 

2350 

2355 

2360 

2366 

2371 

2377 

2382 

2388 

2393 

1 

1 

2 

2 

8 

3 

4 

4 

6 

•38 

2399 

2404 

2410 

2415 

2421 

2427 

2432 

2438 

2443 

2449 

1 

1 

2 

2 

8 

3 

4 

4 

5 

•39 

2455 

2460 

2466 

2472 

2477 

2483 

2489 

2495 

2500 

2506 

1 

1 

2 

2 

3 

3 

4 

5 

5 

•40 

2512 

2518 

2523 

2529 

2535 

2541 

2547 

2553 

2559 

2564 

1 

1 

2 

2 

8 

4 

4 

5 

5 

•41 

2570 

2576 

2582 

2588 

2594 

2600 

2606 

2612 

2618 

2624 

1 

1 

2 

2 

3 

4 

4 

5 

6 

•42 

2630 

2636 

2642 

2649 

2655 

2661 

2667 

2673 

2679 

2685 

1 

1 

2 

2 

3 

4 

4 

5 

6 

•43 

2692 

2698 

2704 

2710 

2716 

2723 

2729 

2735 

2742 

2748 

1 

1 

2 

8 

3 

4 

4 

5 

6 

•44 

2754 

2761 

2767 

2773 

2780 

2786 

2793 

2799 

2805 

2812 

1 

1 

2 

8 

8 

4 

4 

5 

6 

•45 

2818 

2825 

2831 

2838 

2844 

2851 

2858 

2864 

2871 

2877 

1 

1 

2 

8 

8 

4 

5 

5 

6 

•46 

2884 

2891 

2897 

2904 

2911 

2917 

2924 

2931 

2938 

2944 

1 

1 

2 

8 

8 

4 

6 

5 

6 

•47 

2951 

2958 

2965 

2972 

2979 

2985 

2992 

2999 

3006 

8013 

1 

1 

2 

8 

3 

4 

5 

5 

6 

•48 

3020 

3027 

3034 

3041 

3048 

3055 

8062 

3069 

8076 

8083 

1 

1 

2 

8 

4 

4 

5 

6 

6 

•49 

3090 

3097 

3105 

8112 

8119 

3126 

8133 

8141 

8148 

3165 

1 

1 

2 

8 

4 

4 

5 

6 

6 


























































860 


BLBGTRIOAL ENGINRERINa TESTim 


Anti-logarithms. 



0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

1 1 

2 

3 4 

5 

6 

7 

8 9 

•50 

3162 

3170 

3177 

3184 

3192 

3199 

3206 

3214 

3221 

3228 

1 

1 

2 

3 

4 

4 

5 

6 

7 

•51 

3236 

3243 

3251 

8258 

3266 

3273 

3281 

3289 

3296 

3304 

1 

2 

2 

3 

4 

6 

5 

6 

7 

•52 

3311 

8319 

3327 

3334 

3342 

3350 

3357 

3365 

3373 

3381 

1 

2 

2 

3 

4 

5 

5 

6 

7 

•53 

3388 

3396 

3404 

3412 

3420 

3428 

3436 

3443 

3451 

3459 

1 

2 

2 

3 

4 

5 

6 

6 

7 

•54 

8467 

3475 

3483 

3491 

3499 

3508 

3516 

3524 

3532 

3540 

1 

2 

2 

3 

4 

5 

6 

6 

7 

•55 

3548 

3556 

3565 

3573 

3581 

3589 

3597 

3606 

3614 

3622 

1 

2 

2 

3 

4 

5 

6 

7 

7 

•56 

3631 

3639 

3648 

3656 

3664 

3673 

3681 

3690 

3698 

3707 

1 

2 

3 

3 

4 

5 

6 

7 

8 

•57 

3715 

3724 

3733 

3741 

3750 

3758 

3767 

3776 

3784 

3793 

1 

2 

3 

3 

4 

5 

6 

7 

8 

•58 

3802 

3811 

3819 

3828 

3837 

3846 

3855 

3864 

3873 

3882 

1 

2 

3 

4 

4 

5 

6 

7 

8 

•59 

3890 

3899 

3908 

3917 

3926 

3936 

3945 

3954 

3963 

3972 

1 

2 

3 

4 

5 

5 

6 

7 

8 

•60 

3981 

3990 

3999 

4009 

4018 

4027 

4036 

4046 

4055 

4064 

1 

2 

3 

4 

5 

6 

6 

7 

8 

•61 

4074 

4083 

4093 

4102 

4111 

4121 

4130 

4140 

4150 

4159 

1 

2 

3 

4 

5 

6 

7 

8 

9 

•62 

4169 

4178 

4188 

4198 

4207 

4217 

4227 

4236 

4246 

4256 

1 

2 

3 

4 

5 

6 

7 

8 

9 

•63 

4266 

4276 

4285 

4295 

4305 

4315 

4325 

4335 

4345 

4355 

1 

2 

3 

4 

5 

6 

7 

8 

9 

•64 

4365 

4375 

4385 

4395 

4406 

4416 

4426 

4436 

4446 

4457 

1 

2 

3 

4 

5 

6 

7 

8 

9 

•65 

4467 

4477 

4487 

4498 

4508 

4519 

4529 

4539 

4550 

4560 

1 

2 

3 

4 

5 

6 

7 

8 

9 

•66 

4571 

4581 

4592 

4603 

4613 

4624 

4634 

4645 

4656 

4667 

1 

2 

3 

4 

5 

6 

7 

9 

10 

•67 

4677 

4688 

4699 

4710 

4721 

4732 

4742 

4753 

4764 

4775 

1 

2 

3 

4 

5 

7 

8 

9 

10 

•68 

4786 

4797 

4808 

4819 

4831 

4842 

4853 

4864 

4875 

4887 

1 

2 

3 

4 

6 

7 

8 

9 

10 

•69 

4898 

4909 

4920 

4932 

4943 

4955 

4966 

4977 

4989 

5000 

1 

2 

3 

5 

6 

7 

8 

9 

10 

•70 

5012 

5023 

5035 

5047 

5058 

5070 

5082 

5093 

5105 

5117 

1 

2 

4 

5 

6 

7 

8 

9 

11 

•71 

5129 

6140 

5152 

5164 

5176 

5188 

5200 

5212 

5224 

5236 

1 

2 

4 

5 

6 

7 

8 

10 

11 

•72 

5248 

5260 

5272 

5284 

5297 

5309 

5321 

5333 

5346 

5358 

1 

2 

4 

5 

6 

7 

9 

10 

11 

•73 

5370 

5383 

5395 

5408 

5420 

5433 

5445 

5458 

5470 

5*483 

1 

S 

4 

6 

6 

8 

9 

10 

11 

•74 

5495 

6508 

5521 

5534 

5546 

5559 

5572 

5585 

5598 

5610 

1 

O 

4 

5 

6 

8 

9 

10 

12 

•75 

5623 

5636 

5649 

5662 

5675 

5689 

5702 

5715 

5728 

5741 

1 

3 

4 

6 

N 

t 

8 

9 

10 

12 

•76 

5754 

5768 

5781 

5794 

5808 

5821 

5834 

5848 

5861 

5875 

1 

8 

4 

5 

7 

8 

9 

11 

12 

•77 

5888 

5902 

5916 

5929 

5943 

5957 

5970 

5984 

5998 

6012 

1 

8 

4 

5 

7 

8 

10 

11 

12 

•78 

6026 

6039 

6053 

6067 

6081 

6095 

6109 

6124 

6138 

6152 

1 

o 

O 

4 

6 

7 

8 

10 

11 

13 

•79 

6166 

6180 

6194 

6209 

6223 

6237 

6252 

6266 

6281 

6295 

1 

3 

4 

6 

7 

9 

10 

11 

13 

•80 

6310 

6324 

6339 

6353 

6368 

6383 

6397 

6412 

6427 

6442 

1 

3 

4 

6 

t 

9 

10 

12 

13 

•81 

6457 

6471 

6486 

6501 

6516 

6531 

6546 

6561 

6577 

6592 

2 

3 

5 

6 

8 

9 

11 

12 

14 

•82 

6607 

6622 

6637 

6653 

6668 

6683 

6699 

6714 

6730 

6745 

2 

3 

5 

6 

8 

9 

11 

12 

14 

•83 

6761 

6776 

6792 

6808 

6823 

6839 

6855 

6871 

6887 

6902 

2 

3 

5 

6 

8 

9 

11 

13 

14 

•84 

6918 

6934 

6950 

6966 

6982 

6998 

7015 

7031 

7047 

7063 

2 

3 

5 

6 

8 

10 

11 

13 

15 

•85 

7079 

7096 

7112 

7129 

7145 

7161 

7178 

7194 

7211 

7228 

2 

3 

5 

7 

8 

10 

12 

13 

15 

•86 

7244 

7261 

7278 

7295 

7311 

7328 

7345 

7362 

7379 

7396 

2 

3 

5 

7 

8 

10 

12 

13 

15 

•87 

7413 

7430 

7447 

7464 

7482 

7499 

7.516 

7534 

7551 

7568 

2 

3 

5 

7 

9 

10 

12 

14 

16 

•88 

7586 

7603 

7621 

7638 

7656 

7674 

7691 

7709 

7727 

7745 

2 

4 

5 

7 

9 

11 

12 

14 

16 

•89 

7762 

7780 

7798 

7816 

7834 

7852 

7870 

7889 

7907 

7925 

2 

4 

5 

7 

9 

11 

12 

14 

16 

•90 

7943 

7962 

7980 

7998 

8017 

8035 

8054 

8072 

8091 

8110 

2 

4 

6 

7 

9 

11 

13 

15 

17 

•91 

8128 

8147 

8166 

8185 

8204 

8222 

8241 

8260 

8279 

8299 

2 

4 

6 

8 

9 

11 

13 

15 

17 

•92 

8318 

8337 

8356 

8375 

8395 

8414 

8433 

8453 

8472 

8492 

2 

4 

6 

8 

10 

12 

14 

15 

17 

93 

8511 

8531 

8551 

8570 

8590 

8610 

8630 

8650 

8670 

8690 

2 

4 

6 

8 

10 

12 

14 

16 

18 

•94 

8710 

8730 

8750 

8770 

8790 

8810 

8831 

8851 

8872 

8892 

2 

4 

6 

8 

10 

12 

14 

16 

18 

•95 

8913 

8983 

8954 

8974 

8995 

9016 

9036 

9057 

9078 

9099 

2 

4 

6 

8 

10 

12 

15 

17 

19 

•96 

9120 

9141 

9162 

9183 

9204 

9226 

9247 

9268 

9290 

9311 

2 

4 

6 

8 

11 

13 

15 

17 

19 

•97 

9333 

9354 

9376 

9397 

9419 

9441 

9462 

9484 

9506 

9528 

2 

4 

7 

9 

11 

13 

15 

17 

20 

•98 

9550 

9572 

9594 

9616 

9638 

9661 

9683 

9705 

9727 

9750 

2 

4 

7 

9 

11 

13 

16 

18 

20 

•99 

9772 

9795 

9817 

9840 

9863 

9886 

9908 

9931 

9954 

9977 

2 

5 

7 

9 

11 

14 

16 

18 

20 















































ELEGTRIGAL ENGINEERING TESTING 


661 


Squares of Numbers from 1 to 10,000, Correct to Four 
' Significant Figures. 



0 

1 

2 

3 

4 

5 

6 

10 

1000 

1020 

1040 

1061 

1082 

1102 

1124 

11 

1210 

1232 

1254 

1277 

1300 

1323 

1346 

12 

1440 

1464 

1488 

1513 

1538 

1563 

1588 

13 

1690 

1716 

1742 

1769 

1796 

1823 

1850 

14 

1960 

1988 

2016 

2045 

2074 

2103 

2132 

15 

2250 

2280 

2310 

2341 

2372 

2403 

2434 

16 

2660 

2592 

2624 

2657 

2690 

2723 

2756 

17 

2890 

2924 

2958 

2993 

3028 

3063 

3098 

18 

3240 

3276 

3312 

3349 

3386 

3423 

3460 

19 

3610 

3648 

3686 

3725 

3764 

3803 

3842 

20 

4000 

4040 

4080 

4121 

4162 

4203 

4244 

21 

4410 

4452 

4494 

4537 

4580 

4623 

4666 

22 

4840 

4884 

4928 

4973 

5018 

5063 

5108 

23 

5290 

5336 

5382 

5429 

5476 

5523 

5570 

24 

5760 

5808 

5856 

5905 

5954 

6003 

6052 

25 

6250 

6300 

6350 

6401 

6452 

6503 

6554 

26 

6760 

6812 

6864 

6917 

6970 

7023 

7076 

27 

7290 

7344 

7398 

7453 

7508 

7563 

7618 

28 

7840 

7896 

7952 

8009 

8066 

3123 

8180 

29 

8410 

8468 

8526 

8585 

8644 

8703 

8762 

30 

9000 

9060 

9120 

9181 

9242 

9303 

9364 

31 

9610 

9672 

9734 

9797 

9860 

9923 

9986 

32 

1024 

1030 

1037 

1043 

1050 

1056 

1063 

33 

1089 

1096 

1102 

1109 

1116 

1122 

1129 

34 

1156 

1163 

1170 

1176 

1183 

1190 

1197 

35 

1225 

1232 

1239 

1246 

1253 

1260 

1267 

36 

1296 

1303 

1310 

1318 

1325 

1332 

1340 

37 

1369 

1376 

1384 

1391 

1399 

1406 

1414 

38 

1444 

1452 

1459 

1467 

1475 

1482 

1490 

39 

1521 

1529 

1537 

1544 

1552 

1560 

1568 

40 

1600 

1608 

1616 

1624 

1632 

1640 

1648 

41 

1681 

1689 

1697 

1706 

1714 

1722 

1731 

42 

1764 

1772 

1781 

1789 

1798 

1806 

1815 

43 

1849 

1858 

1866 

1875 

1884 

1892 

1901 

44 

1936 

1945 

1954 

1962 

1971 

1980 

1989 

45 

2025 

2034 

2043 

• 

2052 

2061 

2070 

2079 

46 

2116 

2125 

2134 

2144 

2153 

2162 

2172 

47 

2209 

2218 

2228 

2237 

2247 

2256 

2266 

48 

2304 

2314 

2323 

2333 

2343 

2352 

2362 

49 

2401 

2411 

2421 

2430 

2440 

2450 

2460 

50 

2500 

2510 

2520 

2530 

2540 

2550 

2560 

51 

2601 

2611 

2621 

2632 

2642 

2652 

2663 

52 

2704 

2714 

2725 

2735 

2746 

2756 

2767 

53 

2809 

2820 

2830 

2841 

2852 

2862 

2873 

54 

2916 

2927 

2938 

2948 

2959 

2970 

2981 


7 

8 

9 

X 

2 

3 

4 

5 

6 

7 

8 

9 

1145 

1166 

1188 

3 

5 

7 

9 

11 

13 

15 

17 

19 

1369 

1392 

1416 

3 

5 

7 

10 

12 

14 

17 

19 

21 

1613 

1638 

1664 

3 

5 

8 

10 

13 

15 

18 

20 

23 

1877 

1904 

1932 

3 

6 

9 

11 

14 

17 

19 

22 

25 

2161 

2190 

2220 

3 

6 

9 

12 

15 

18 

21 

24 

27 

2465 

2496 

2528 

4 

7 

10 

13 

16 

19 

22 

25 

28 

2789 

2822 

2856 

4 

7 

10 

14 

17 

20 

24 

27 

30 

3133 

3168 

3204 

4 

7 

11 

14 

18 

21 

25 

28 

32 

3497 

3534 

3572 

4 

8 

12 

15 

19 

23 

26 

30 

34 

3881 

3920 

3960 

4 

8 

12 

16 

20 

24 

28 

32 

36 

4285 

4326 

4368 

5 

9 

13 

17 

21 

25 

29 

33 

37 

4709 

4752 

4796 

5 

9 

13 

18 

22 

26 

31 

34 

39 

5153 

5198 

5244 

5 

9 

14 

18 

23 

27 

32 

36 

41 

5617 

5664 

5712 

5 

10 

15 

19 

24 

27 

33 

38 

43 

6101 

6150 

6200 

5 

10 

15 

20 

25 

30 

35 

40 

45 

6605 

6656 

6708 

6 

11 

16 

21 

26 

31 

36 

41 

46 

7129 

7182 

7236 

6 

11 

16 

22 

27 

32 

38 

43 

48 

7673 

7728 

7784 

6 

11 

17 

22 

28 

33 

39 

44 

50 

8237 

8294 

8352 

6 

12 

18 

23 

29 

35 

40 

46 

52 

8821 

8880 

8940 

6 

12 

18 

24 

30 

36 

42 

48 

54 

9425 

9486 

9548 

7 

13 

19 

25 

31 

37 

43 

49 

55 

10051 

10111 

10181 

7 

13 

19 

26 

32 

38 

45 

51 

57 

1069 

1076 

1082 

1 

1 

2 

3 

3 

4 

5 

5 

6 

1136 

1142 

1149 

1 

1 

2 

3 

4 

4 

5 

6 

6 

1204 

1211 

1218 

1 

2 

2 

3 

4 

4 

5 

6 

6 

1273 

1282 

1289 

1 

2 

2 

3 

4 

4 

5 

6 

7 

1347 

1354 

1362 

1 

2 

2 

3 

4 

5 

6 

6 

7 

1421 

1429 

1436 

1 

2 

2 

3 

4 

5 

5 

6 

7 

1498 

1505 

1513 

1 

2 

2 

3 

4 

5 

6 

6 

7 

1576 

1584 

1592 

1 

2 

3 

3 

4 

5 

6 

6 

7 

1656 

1665 

1673 

1 

2 

3 

3 

4 

5 

6 

7 

7 

1739 

1747 

1756 

1 

2 

3 

3 

4 

5 

6 

7 

8 

1823 

1832 

1840 

1 

2 

3 

4 

4 

5 

6 

7 

8 

1910 

1918 

1927 

1 

2 

3 

4 

6 

5 

6 

7 

8 

1998 

2007 

2016 

1 

2 

3 

4 

5 

5 

6 

7 

8 

2088 

2098 

2107 

1 

2 

3 

4 

5 

6 

7 

7 

8 

2181 

2190 

2200 

1 

2 

3 

4 

5 

6 

7 

8 

9 

2275 

2285 

2294 

1 

2 

3 

4 

5 

6 

7 

8 

9 

2372 

2381 

2391 

1 

2 

3 

4 

5 

6 

7 

8 

9 

2470 

2480 

2490 

1 

2 

3 

4 

5 

6 

7 

8 

9 

2570 

2581 

2591 

1 

2 

3 

4 

5 

6 

7 

8 

9 

2673 

2683 

2694 

1 

2 

3 

4 

5 

6 

7 

8 

9 

2777 

2788 

2798 

1 

2 

3 

4 

5 

6 

8 

9 

10 

2884 

2894 

2905 

1 

2 

3 

4 

6 

7 

8 

9 

10 

2992 

3003 

3014 

1 

2 

3 

5 

6 

7 

8 

9 

10 


Squares from 1 to 3 contain 1 figure. 
„ „ 4 to 9 „ 2 figures. 

,, M 19 to 31 ,, 3 ,, 

„ 32 to 99 „ 4 ,, 


Squares from 100 to 316 contain 5 figures. 
„ „ 317 to 999 „ 6 „ 

„ „ 1000 to 3162 „ 7 ,, 

„ „ 3163 to 10000 „ 8 ,, 


The differences for squares from 3171 to 3199 are 1, 1, 2, 3, 3, 4, 5, 5, 6. 


















































662 


ELECTRICAL ENGINEERING TESTING 


Squares op Numbers from 1 to 10,000, Correct to Four 

Significant Figures. 



0 

1 

2 

3 

4 

6 

6 

7 

8 

9 

1 

2 

3 

4 

5 

6 

7 

8 

9 

55 

8025 

8036 

3047 

3058 

3069 

3080 

8091 

8102 

8114 

8125 

1 

2 

3 

5 

6 

r 

8 

9 

10 

56 

8136 

3147 

3158 

3170 

3181 

3192 

3204 

3215 

8226 

8238 

1 

2 

4 

5 

6 

7 

8 

9 

10 

67 

3249 

3260 

3272 

3283 

3295 

3306 

3318 

3329 

8341 

8352 

1 

2 

4 

6 

6 

7 

8 

9 

11 

58 

8364 

3376 

3387 

3399 

3411 

3422 

3434 

3446 

3457 

3469 

1 

2 

4 

5 

6 

7 

8 

10 

11 

59 

3481 

3493 

8505 

3516 

3528 

3540 

3552 

3564 

8576 

8588 

1 

8 

4 

6 

6 

7 

8 

10 

11 

60 

8600 

3612 

3624 

3636 

3648 

3660 

3672 

3684 

3697 

3709 

1 

3 

4 

5 

6 

7 

9 

10 

11 

61 

3721 

3733 

3745 

3758 

3770 

3782 

3795 

3807 

8819 

3832 

1 

8 

4 

6 

6 

8 

9 

10 

11 

62 

3844 

3856 

3869 

3881 

3894 

3906 

3919 

3931 

3944 

3956 

1 

8 

4 

5 

6 

8 

9 

10 

11 

63 

8969 

3982 

3994 

4007 

4020 

4032 

4045 

4058 

4070 

4083 

1 

8 

4 

5 

7 

8 

9 

10 

12 

64 

4096 

4109 

4122 

4134 

4147 

4160 

4173 

4186 

4199 

4212 

1 

3 

4 

5 

7 

8 

9 

10 

12 

65 

4225 

4238 

4251 

4264 

4277 

4290 

4303 

4316 

4330 

4343 

1 

3 

4 

5 

7 

8 

9 

11 

12 

66 

4856 

4369 

4382 

4396 

4409 

4422 

4486 

4449 

4462 

4476 

1 

3 

4 

5 

7 

8 

9 

11 

12 

67 

4489 

4502 

4516 

4529 

4543 

4556 

4570 

4583 

4597 

4610 

2 

8 

4 

6 

7 

8 

10 

11 

12 

68 

4624 

4638 

4651 

4665 

4679 

4692 

4706 

4720 

4733 

4747 

2 

8 

4 

6 

7 

8 

10 

11 

12 

69 

4761 

4775 

4789 

4602 

4816 

4830 

4844 

4858 

4872 

4886 

2 

3 

4 

6 

7 

8 

10 

11 

13 

70 

4900 

4914 

4928 

4942 

4956 

4970 

4984 

4998 

5013 

6027 

2 

3 

4 

6 

7 

9 

10 

11 

13 

71 

5041 

5055 

5069 

5084 

5098 

5112 

5127 

6141 

6155 

5170 

2 

8 

4 

6 

7 

9 

10 

12 

13 

72 

5184 

5198 

5213 

5227 

5242 

5256 

6271 

5285 

5300 

6314 

2 

3 

5 

6 

7 

9 

10 

12 

13 

73 

5329 

6344 

5358 

5373 

6388 

5402 

6417 

5432 

5446 

5461 

2 

3 

5 

6 

8 

9 

10 

12 

13 

74 

5476 

5491 

6506 

6520 

5535 

5550 

6565 

5580 

5595 

5610 

2 

8 

5 

6 

8 

9 

11 

12 

14 

76 

6625 

6640 

5655 

5670 

6685 

5700 

5716 

5730 

5746 

6761 

2 

3 

6 

6 

8 

9 

11 

12 

14 

76 

5776 

5791 

5806 

5822 

6837 

6852 

5868 

5883 

5898 

5914 

2 

3 

5 

6 

8 

9 

11 

12 

14 

77 

5929 

5944 

5960 

5975 

5991 

6006 

6022 

6037 

6053 

6068 

2 

3 

5 

6 

8 

9 

11 

13 

14 

78 

6084 

6100 

6115 

6131 

6147 

6162 

6178 

6194 

6209 

6225 

2 

8 

5 

6 

8 

10 

11 

13 

14 

79 

6241 

6257 

6273 

6288 

6304 

6320 

6336 

6352 

6368 

6384 

2 

3 

5 

7 

8 

10 

11 

13 

14 

80 

6400 

6416 

6432 

6448 

6464 

6480 

6496 

6612 

6529 

6545 

2 

3 

5 

7 

8 

10 

11 

13 

15 

81 

6561 

6577 

6593 

6610 

6626 

6642 

6659 

6675 

6691 

6708 

2 

3 

5 

7 

8 

10 

12 

13 

15 

82 

6724 

6740 

6757 

6773 

6790 

6806 

6823 

6839 

6856 

6872 

2 

3 

5 

7 

8 

10 

12 

13 

15 

83 

6889 

6906 

6922 

6939 

6956 

6972 

6989 

7006 

7022 

7039 

2 

3 

5 

7 

9 

10 

12 

14 

15 

84 

7056 

7073 

7090 

7106 

7123 

7140 

7157 

7174 

7191 

7208 

2 

4 

6 

7 

9 

10 

12 

14 

15 

85 

7225 

7242 

7259 

7276 

7293 

7310 

7327 

7344 

7362 

7379 

2 

4 

5 

7 

9 

10 

12 

14 

16 

86 

7396 

7413 

7430 

7448 

7465 

7482 

7600 

7517 

7634 

7652 

2 

4 

5 

7 

9 

11 

12 

14 

16 

87 

7569 

7686 

7604 

7621 

7639 

7656 

7674 

7691 

7709 

7726 

2 

4 

5 

7 

9 

11 

12 

14 

16 

88 

7744 

7762 

7779 

7797 

7815 

7832 

7850 

7868 

7885 

7903 

2 

4 

5 

7 

9 

11 

13 

14 

16 

89 

7921 

7939 

7967 

7974 

7992 

8010 

8028 

8046 

8064 

8082 

2 

4 

6 

7 

9 

11 

13 

14 

16 

90 

8100 

8118 

8136 

8154 

8172 

8190 

8208 

8226 

8245 

8263 

2 

4 

6 

7 

9 

11 

13 

15 

16 

91 

8281 

8299 

8317 

8336 

8354 

8372 

8391 

8409 

8427 

8446 

2 

4 

6 

7 

9 

11 

13 

15 

17 

92 

8464 

8482 

8501 

8519 

8538 

8556 

8575 

8593 

8612 

8630 

2 

4 

6 

8 

9 

11 

13 

15 

17 

93 

8649 

8668 

8686 

8705 

8724 

8742 

8761 

8780 

8798 

8817 

2 

4 

6 

8 

10 

11 

13 

15 

17 

94 

8836 

8855 

8874 

8892 

8911 

8930 

8949 

8968 

8987 

9006 

2 

4 

6 

8 

10 

11 

13 

15 

17 

95 

9025 

9044 

9063 

9082 

9101 

9120 

9139 

9168 

9178 

9197 

2 

4 

6 

8 

10 

12 

14 

15 

17 

96 

9216 

9235 

9254 

9274 

9293 

9312 

9332 

9351 

9370 

9390 

2 

4 

6 

8 

10 

12 

14 

16 

18 

97 

9409 

9428 

9448 

9467 

9487 

9505 

9526 

9545 

9565 

9584 

2 

4 

6 

8 

10 

12 

14 

16 

18 

98 

9604 

9624 

9643 

9663 

9683 

9702 

9722 

9742 

9761 

9781 

2 

4 

6 

8 

10 

12 

14 

16 

18 

99 

9801 

9821 

9841 

9860 

9880 

9900 

9920 

9940 

9960 

9980 

2 

4 

6 

8 

10 

12 

14 

16 

18 


Squares from 1 to 3 contain 1 figure. 


Squares from 100 to 



M 

4 to 9 

ij 

2 figures. 


tt 

317 to 999 


6 

t) 

fy 

10 to 31 


3 „ 


tt 

1000 to 3162 

9 9 

7 

)i 

»} 

32 to 99 

tt 

4 ,, 

tt 

tt 

3103 to 10000 

tt 

8 


tt 

tf 































ELEOTEICAL ENOINEEBINO TESTING 


663 


Reciprocals of Numbers from 1 to 9999. 



0 

1 

2 

3 

1 

5 

6 

7 

8 

9 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

0000 

9901 

9804 

9709 

9615 

9524 

9434 

9346 

9259 

9174 

9 18 

27 36 

45 

54 

63 

72 

81 

11 

9091 

9009 

8929 

8850 

8772 

8696 

8621 

8547 

8475 

8403 

8 16 

23 31 

38 

46 

63 

61 

68 

12 

8333 

8264 

8197 

8130 

8065 

8000 

7937 

7874 

7813 

7752 

6 

13 

19 26 

32 

38 

45 

51 

57 

13 

7692 

7634 

7576 

7519 

7463 

7407 

7353 

7299 

7246 

7194 

5 

11 

16 22 

27 

32 

38 

43 

49 

14 

7143 

7092 

7042 

6993 

6944 

6897 

6849 

6803 

6767 

6711 

5 

9 

14 

19 

23 

28 

33 

37 

42 

15 

6667 

6623 

6579 

6536 

6494 

6452 

6410 

6369 

6329 

6289 

5 

9 

13 17 

21 

25 

29 

34 

38 

16 

6250 

6211 

6173 

6135 

6098 

6061 

6024 

5988 

6952 

5917 

4 

8 

11 

15 

19 

22 

26 

30 

33 

17 

6882 

5848 

5814 

5780 

5747 

5714 

5682 

6650 

5618 

6587 

3 

6 

10 13 

16 

19 

23 

26 

29 

18 

6556 

5525 

5495 

5464 

5435 

5405 

5376 

5348 

5319 

5291 

3 

6 

8 

11 

14 

17 

20 

23 

26 

19 

5263 

5236 

6208 

5181 

5155 

5128 

5102 

5076 

5051 

5025 

3 

5 

8 10 

18 

16 

18 

21 

23 

20 

6000 

4975 

4950 

4926 

4902 

4878 

4854 

4831 

4808 

4785 

2 

5 

7 10 

12 

14 

17 

19 

21 

21 

4762 

4739 

4717 

4695 

4673 

4651 

4630 

4608 

4587 

4566 

2 

4 

6 

9 

11 

13 

15 

17 

19 

22 

4545 

4525 

4505 

4484 

4464 

4444 

4425 

4405 

4386 

4367 

2 

4 

6 

8 

10 

11 

13 

15 

17 

23 

4348 

4329 

4310 

4292 

4274 

4255 

4237 

4219 

4202 

4184 

2 

3 

5 

7 

9 

11 

12 

14 

16 

24 

4167 

4149 

4132 

4115 

4098 

4082 

4065 

4049 

4032 

4016 

2 

4 

5 

7 

9 

10 

12 

14 

15 

25 

4000 

3984 

3968 

3953 

3937 

3921 

3906 

3891 

3876 

8861 

1 

3 

4 

6 

7 

9 

10 

12 

13 

26 

8846 

3831 

3817 

3802 

3788 

3774 

3759 

3745 

3731 

3717 

1 

2 

4 

5 

7 

8 

9 

11 

12 

27 

8704 

3690 

3676 

3663 

3650 

3636 

3623 

3610 

3597 

3584 

1 

2 

4 

5 

6 

8 

9 

10 

12 

28 

3571 

3559 

3546 

3534 

3521 

3509 

3497 

3484 

3472 

3460 

2 

3 

4 

5 

6 

8 

9 

10 

11 

29 

3448 

3436 

3425 

3413 

3401 

3390 

3378 

3367 

3356 

3344 

1 

3 

4 

5 

6 

7 

8 

9 

11 

SO 

8333 

3322 

3311. 

3300 

3289 

3279 

3268 

3257 

3247 

3237 

1 

3 

4 

6 

6 

7 

8 

9 

10 

31 

8226 

3215 

3205 

3195 

3185 

3175 

3165 

3155 

3145 

3135 

2 

3 

4 

5 

6 

7 

8 

9 

10 

32 

3125 

3115 

3106 

3096 

3086 

3077 

3067 

3058 

3049 

8040 

1 

2 

3 

4 

5 

6 

7 

8 

9 

33 

3030 

3021 

3012 

3003 

2994 

2985 

2976 

2967 

2959 

2950 

1 

2 

3 

4 

4 

5 

6 

7 

8 

34 

2941 

2933 

2924 

2915 

2907 

2899 

2890 

2882 

2874 

2865 

0 

1 

2 

3 

4 

5 

5 

6 

7 

35 

2857 

2849 

2841 

2833 

2825 

2817 

2809 

2801 

2793 

2785 

1 

2 

3 

3 

4 

5 

6 

7 

7 

36 

2778 

2770 

2762 

2755 

2747 

2740 

2732 

2725 

2717 

2710 

1 

2 

3 

3 

4 

5 

6 

6 

7 

37 

2703 

2695 

2688 

2681 

2674 

2667 

2660 

2653 

2646 

2639 

1 

2 

3 

3 

4 

5 

5 

6 

7 

38 

2632 

2625 

2618 

2611 

2604 

2597 

2591 

2584 

2577 

2571 

0 

1 

2 

2 

3 

4 

4 

5 

6 

39 

2564 

2558 

2551 

2545 

2538 

2532 

2525 

2519 

2513 

2506 

1 

2 

2 

3 

4 

4 

5 

6 

6 

40 

2500 

2494 

2488 

2481 

2475 

2469 

2463 

2457 

2451 

2445 

1 

1 

2 

2 

3 

4 

4 

5 

5 

41 

2439 

2433 

2427 

2421 

2415 

2410 

2404 

2398 

2392 

2387 

1 

2 

2 

3 

3 

4 

5 

5 

6 

42 

2381 

2375 

2370 

2364 

2358 

2353 

2347 

2342 

2336 

2331 

1 

1 

2 

2 

3 

3 

4 

6 

5 

43 

2326 

2320 

2316 

2309 

2304 

2299 

2294 

2288 

2283 

2278 

1 

1 

2 

2 

3 

3 

4 

4 

5 

44 

2273 

2268 

2262 

2257 

2252 

2247 

2242 

2237 

2232 

2227 

0 

1 

1 

2 

2 

3 

3 

4 

4 

45 

2222 

2217 

2212 

2208 

2203 

2198 

2193 

2188 

2183 

2179 

1 

1 

2 

2 

3 

3 

4 

4 

6 

46 

2174 

2169 

2165 

2160 

2155 

2151 

2146 

2141 

2137 

2132 

0 

0 

1 

1 

2 

2 

3 

3 

4 

47 

2128 

2123 

2119 

2114 

2110 

2105 

2101 

2096 

2092 

2088 

0 

1 

1 

2 

2 

2 

3 

3 

4 

48 

2083 

2079 

2075 

2070 

2066 

2062 

2058 

2053 

2049 

2045 

1 

1 

2 

2 

2 

3 

3 

4 

4 

49 

2041 

2037 

2033 

2028 

2024 

2020 

2016 

2012 

2008 

2004 

0 

1 

1 

2 

2 

2 

3 

3 

4 

50 

2000 

1996 

1992 

1988 

1984 

1980 

1976 

1972 

1969 

1965 

0 

1 

1 

1 

2 

2 

3 

3 

3 

51 

1961 

1957 

1953 

1949 

1946 

1942 

1938 

1934 

1931 

1927 

1 

1 

1 

2 

2 

3 

3 

3 

4 

52 

1923 

1919 

1916 

1912 

1908 

1905 

1901 

1898 

1894 

1890 

1 

1 

1 

2 

2 

3 

3 

3 

4 

53 

1887 

1883 

1880 

1876 

1873 

1869 

1866 

1862 

1859 

1855 

0 

1 

1 

1 

2 

2 

2 

3 

3 

54 

1852 

1848 

1845 

1842 

1 

1838 

1835 

1832 

1828 

1825 

1821 

1 

1 

1 

2 

2 

2 

3 

3 

3 


Reciprocals from 2 to 10=0- I Reciprocals from W1 to 1000 = 0-W 

„ „ to 100=0*0 I II II 1001 to 9999=0-000 

Note.— Numbers in difference columns to be subtracted, not added. 














































064 


ELECTRICAL ENGINEERING TESTING 


Keciprocals of Numbers from 1 to 9999 . 



0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

1 

2 

3 

4 

5 

6 

7 

8 

9 

55 

1818 

1815 

1812 

1808 

1805 

1802 

1799 

1795 

1792 

1789 

1 

1 

1 

2 

2 

2 

3 

3 

3 

56 

1786 

1783 

1779 

1776 

1773 

1770 

1767 

1764 

1761 

1757 

1 

1 

1 

1 

2 

2 

2 

3 

3 

57 

1754 

1751 

1748 

1745 

1742 

1739 

1736 

1733 

1730 

1727 

0 

1 

1 

1 

1 

2 

2 

2 

3 

58 

1724 

1721 

1718 

1715 

1712 

1709 

1706 

1704 

1701 

1698 

0 

0 

1 

1 

1 

1 

2 

2 

2 

59 

1695 

1692 

1689 

1686 

1684 

1681 

1678 

1675 

1672 

1669 

1 

1 

1 

2 

2 

2 

2 

3 

3 

60 

1667 

1664 

1661 

1658 

1656 

1653 

1650 

1647 

1645 

1642 

0 

1 

1 

1 

2 

2 

2 

2 

3 

61 

1639 

1637 

1634 

1631 

1629 

1626 

1623 

1621 

1618 

1616 

0 

1 

1 

1 

1 

2 

2 

2 

2 

62 

1613 

1610 

1608 

1605 

1603 

1600 

1597 

1595 

1592 

1590 

0 

1 

1 

1 

1 

2 

2 

2 

2 

63 

1587 

1585 

1582 

1580 

1577 

1575 

1572 

1570 

1567 

1565 

1 

1 

1 

1 

2 

2 

2 

2 

3 

64 

1563 

1560 

1558 

1555 

1553 

1550 

1548 

1546 

1543 

1541 

0 

0 

0 

1 

1 

1 

1 

2 

2 

65 

1538 

1536 

1534 

1531 

1529 

1527 

1524 

1522 

1520 

1517 

1 

1 

1 

1 

2 

2 

2 

2 

2 

66 

1515 

1513 

1511 

1508 

1506 

1504 

1502 

1499 

1497 

1495 

1 

1 

1 

1 

1 

2 

2 

2 

2 

67 

1493 

1490 

1488 

1486 

1484 

1481 

1479 

1477 

1475 

1473 

0 

0 

0 

0 

1 

1 

1 

1 

2 

68 

1471 

1468 

1466 

1464 

1462 

1460 

1458 

1456 

1453 

1451 

0 

1 

1 

1 

1 

2 

2 

2 

2 

69 

1449 

1447 

1445 

1443 

1441 

1439 

1437 

1435 

1433 

1431 

0 

1 

1 

1 

1 

1 

2 

2 

2 

70 

1428 

1427 

1425 

1422 

1420 

1418 

1416 

1414 

1412 

1410 

0 

0 

0 

0 

1 

1 

1 

1 

1 

71 

1408 

1406 

1404 

1403 

1401 

1399 

1397 

1395 

1393 

1391 

1 

1 

1 

1 

1 

2 

2 

2 

2 

72 

1389 

1387 

1385 

1383 

1381 

1379 

1377 

1376 

1374 

1372 

0 

0 

0 

1 

1 

1 

1 

1 

1 

73 

1370 

1368 

1366 

1364 

1362 

1361 

1359 

1357 

1355 

1353 

0 

0 

0 

0 

0 

1 

1 

1 

1 

74 

1351 

1350 

1348 

1346 

1344 

1342 

1340 

1339 

1337 

1335 

0 

0 

0 

1 

1 

1 

1 

1 

1 

75 

1333 

1332 

1330 

1328 

1326 

1325 

1323 

1321 

1319 

1317 

0 

0 

0 

0 

0 

1 

1 

1 

1 

76 

1316 

1314 

1312 

1311 

1309 

1307 

1305 

1304 

1302 

1300 

0 

0 

0 

1 

1 

1 

1 

1 

1 

77 

1299 

1297 

1295 

1294 

1292 

1290 

1289 

1287 

1285 

1284 

0 

0 

0 

0 

1 

1 

1 

1 

1 

78 

1282 

1280 

1279 

1277 

1276 

1274 

1272 

1271 

1269 

1267 

0 

1 

1 

1 

1 

1 

1 

2 

2 

79 

1266 

1264 

1263 

1261 

1259 

1258 

1256 

1255 

1253 

1252 

0 

1 

1 

1 

1 

1 

1 

2 

2 

80 

1250 

1248 

1247 

1245 

1244 

1242 

1241 

1239 

1238 

1236 

0 

0 

0 

0 

1 

1 

1 

1 

1 

81 

1235 

1233 

1232 

1230 

1229 

1227 

1225 

1224 

1222 

1221 

0 

0 

1 

1 

1 

1 

1 

1 

1 

82 

1220 

'1218 

1217 

1215 

1214 

1212 

1211 

1209 

1208 

1206 

0 

0 

0 

1 

1 

1 

1 

1 

1 

83 

1205 

1203 

1202 

1200 

1199 

1198 

1196 

1195 

1193 

1192 

1 

1 

1 

1 

1 

1 

1 

2 

2 

84 

1190 

1189 

1188 

1186 

1185 

1183 

1182 

1181 

1179 

1178 

0 

0 

0 

0 

0 

1 

1 

1 

1 

85 

1176 

1175 

1174 

1172 

1171 

1170 

1168 

1167 

1166 

1164 

0 

0 

0 

0 

0 

0 

0 

1 

1 

86 

1163 

1161 

1160 

1159 

1157 

1156 

1155 

1153 

1152 

1151 

0 

0 

0 

1 

1 

1 

1 

1 

1 

87 

1149 

1148 

1147 

1145 

1144 

1143 

1142 

1140 

1139 

1138 

0 

1 

1 

1 

1 

1 

1 

1 

1 

88 

1136 

1135 

1134 

1133 

1131 

1130 

1129 

1127 

1126 

1125 

0 

0 

1 

1 

1 

1 

1 

1 

1 

89 

1124 

1122 

1121 

1120 

1119 

1117 

1116 

1115 

1114 

1112 

0 

0 

0 

0 

0 

1 

1 

1 

1 

90 

1111 

1110 

1109 

1107 

1106 

1105 

1104 

1103 

1101 

1100 

0 

0 

0 

1 

1 

1 

1 

1 

1 

91 

1099 

1098 

1096 

1095 

1094 

1093 

1092 

1091 

1089 

1088 

0 

0 

1 

1 

1 

1 

1 

1 

1 

92 

1087 

1086 

1085 

1083 

1082 

1081 

1080 

1079 

1078 

1076 

0 

0 

0 

0 

1 

1 

1 

1 

1 

93 

1075 

1074 

1073 

1072 

1071 

1070 

1068 

1067 

1066 

1065 

0 

0 

0 

0 

0 

0 

0 

0 

1 

94 

1064 

1063 

1062 

1060 

1059 

1058 

1057 

1056 

1055 

1054 

0 

0 

0 

0 

0 

1 

1 

1 

1 

95 

1053 

1052 

1050 

1049 

1048 

1047 

1046 

1045 

1044 

1043 

0 

0 

0 

0 

1 

1 

1 

1 

1 

96 

1042 

1041 

1040 

1038 

1037 

1036 

1035 

1034 

1033 

1032 

0 

0 

0 

0 

0 

0 

1 

1 

1 

97 

1031 

1030 

1029 

1028 

1027 

1026 

1025 

1024 

1022 

1021 

1 

1 

1 


1 

1 

1 

1 

1 

98 

1020 

1019 

1018 

1017 

1016 

1015 

1014 

1013 

1012 

1011 

0 

0 

0 

0 

0 

0 

1 

1 

1 

99 

1010 

1009 

1008 

1007 

1006 

1005 

1004 

1003 

1002 

1001 

0 

0 

0 

0 

1 

i 

1 

1 

1 


Reciprocals from 2 to 10 = 0 " j Reciprocals from 101 to 1000=0 00 

„ „ 11 to 100=0-0 I „ ,, 1001 to 9999=0 000 

Note.— Numbers in difference columns to be subtracted, not added. 








































ELECTRICAL ENGINEERING TESTING 


665 


Table op Doubled Square Roots for 



0 

100 

200 

300 

400 

500 

600 

700 

800 

900 


0 

0-000 

20-00 

28-28 

34-64 

40-00 

44-72 

48-99 

52-92 

56-57 

60-00 

0 

1 

2-000 

20-10 

28-35 

34-70 

40-05 

44-77 

49-03 

52-95 

56-60 

60-03 

1 

2 

2-828 

20-20 

28-43 

34-76 

40-10 

44-81 

49-07 

52-99 

56-64 

60-07 

2 

3 

3-464 

20-30 

28-50 

34-81 

40-15 

44-86 

49-11 

53-03 

56-67 

60-10 

3 

4 

4-000 

20-40 

28-57 

34-87 

40-20 

44-90 

49-15 

53-07 

56-71 

60-13 

4 

5 

4-472 

20-49 

28-64 

34-93 

40-25 

44-94 

49-19 

53-10 

56-75 

60-17 

6 

6 

4-899 

20-59 

28-71 

34-99 

40-30 

44-99 

49-23 

53-14 

56-78 

60-20 

6 

7 

5-292 

20-69 

28-77 

35-04 

40-35 

45-03 

49-27 

53-18 

56-82 

60-23 

7 

8 

5-657 

20-78 

28-84 

35-10 

40-40 

45-08 

49-32 

53-22 

56-85 

60-27 

8 

9 

6-000 

20-88 

28-91 

35-16 

40-45 

45-12 

49-36 

53-25 

56-89 

60-30 

9 

10 

6-325 

20-98 

28-98 

35-21 

40-50 

45-17 

49-40 

53-29 

56-92 

60-33 

10 

11 

6-633 

21-07 

29-05 

35-27 

40-55 

45-21 

49-44 

53-33 

56-96 

60-37 

11 

12 

6-928 

21-17 

29-12 

35-33 

40-60 

45-25 

49-48 

53-37 

56-99 

60-40 

12 

13 

7-211 

21-26 

29-19 

35-38 

40-64 

45-30 

49-52 

53-40 

57-03 

60-43 

13 

14 

7-483 

21-35 

29-26 

35-44 

40-69 

45-34 

49-56 

53-44 

57-06 

60-46 

14 

15 

7-746 

21-45 

29-33 

35-50 

40-74 

45-39 

49-60 

53-48 

57-10 

60-50 

15 

16 

8-000 

21-54 

29-39 

35-55 

40-79 

45-43 

49-64 

53-52 

57-13 

60-53 

16 

17 

8-246 

21-63 

29-46 

35-61 

40-84 

45-48 

49-68 

53-55 

57-17 

60 56 

17 

18 

8-485 

21-73 

29-53 

35-67 

40-89 

45-52 

49-72 

53-59 

57-20 

60-60 

18 

19 

8-718 

21-82 

29-60 

35-72 

40-94 

45-56 

49-76 

53-63 

57-24 

60-63 

19 

20 

8-944 

21-91 

29-66 

35-78 

40-99 

45-61 

49-80 

53-67 

57-27 

60-66 

20 

21 

9-165 

22 00 

29-73 

85-83 

41-04 

45-65 

49-84 

53-70 

57-31 

60-70 

21 

22 

9-381 

22 09 

29-80 

35-89 

41-09 

45-69 

49-88 

53-74 

57-34 

60-73 

22 

23 

9-592 

22-18 

29-87 

35-94 

41-13 

45-74 

49-92 

53-78 

57-38 

60-76 

23 

24 

9-798 

22-27 

29-93 

36-00 

41-18 

45-78 

49-96 

53-81 

57-41 

60-79 

24 

25 

10-000 

22-36 

30-00 

36-06 

41-23 

45-83 

50-00 

53-85 

57-45 

60-83 

25 

26 

10-198 

22-45 

30-07 

36-11 

41-28 

45-87 

50-04 

53-89 

57-48 

60-86 

26 

27 

10-392 

22-54 

30-13 

36-17 

41-33 

45-91 

50-08 

53-93 

57-52 

60-89 

27 

28 

10-583 

22-63 

30-20 

36-22 

41-38 

45-96 

50-12 

53-96 

57-55 

60-93 

28 

29 

10-770 

22-72 

30-27 

36-28 

41-42 

46-00 

50-16 

54-00 

57-58 

60-96 

29 

30 

10-954 

22-80 

30-33 

36-33 

41-47 

46-04 

50-20 

54-04 

57-62 

60-99 

30 

31 

11-136 

22-89 

30-40 

36-89 

41-52 

46-09 

50-24 

54-07 

57-65 

61-02 

31 

32 

11-314 

22-98 

30-46 

36-44 

41-57 

46-13 

50-28 

54-11 

57-69 

61-06 

32 

33 

11-489 

23-07 

30-53 

36-50 

41-62 

46-17 

50-32 

5415 

57-72 

61-09 

33 

34 

11-662 

23-15 

30-59 

36-55 

41-67 

46-22 

50-36 

54-18 

57-76 

61-12 

34 

35 

11-832 

23-24 

30-66 

36-61 

41-71 

46-26 

50-40 

54-22 

57-79 

61-16 

35 

36 

12-000 

23-82 

80-72 

36-66 

41-76 

46-30 

50-44 

54-26 

57-83 

61-19 

36 

87 

12-166 

23-41 

30-79 

36-72 

41-81 

46-35 

50-48 

54-30 

57-86 

61-22 

37 

38 

12-329 

23-49 

30-85 

36-77 

41-86 

46-39 

50-52 

54-33 

57-90 

61-25 

38 

39 

12-490 

23-58 

30-92 

36-82 

41-90 

46-43 

50-56 

54-37 

57-93 

61-29 

39 

40 

12-649 

23-66 

30-98 

36-88 

41-95 

46-48 

50-60 

54-41 

57-97 

61-32 

40 

41 

12-806 

23-75 

81-05 

36-93 

42-00 

46-52 

50-64 

54-44 

58-00 

61-35 

41 

42 

12-961 

23-83 

31-11 

36-99 

42-05 

46-56 

50-68 

54-48 

58-03 

61-38 

42 

43 

13-115 

23-92 

31-18 

37-04 

42-10 

46-60 

50-71 

54-52 

58-07 

61-42 

43 

44 

13-266 

24 00 

31-24 

37-09 

42-14 

46-65 

50-75 

54-55 

58-10 

61 -45 

44 

45 

13-416 

24 08 

31-30 

3715 

42-19 

46-69 

50-79 

54*59 

58-14 

64-48 

45 

46 

13-565 

24-17 

31-37 

37-20 

42-24 

46-73 

50-83 

54-63 

58-17 

61-51 

46 

47 

13-711 

24-25 

31-43 

37-26 

42-28 

46-78 

50-87 

54-66 

58-21 

61 -55 

47 

48 

13-856 

24-33 

31-50 

37 31 

42-33 

46-82 

50-91 

54-70 

58-24 

61-58 

48 

49 

14-000 

24-41 

31-56 

37-36 

42-38 

46-86 

50-95 

54-74 

58-28 

61-61 

49 

50 

14-142 

24-49 

31-62 

37-42 

42-43 

46-90 

50-99 

54-77 

58-31 

61 64 

50 
















































































































































666 


ELECTRICAL ENGINEERING TESTING 


Lord Kelvin’s Standard Electric Balances. 



0 

100 

200 

300 

400 

500 

600 

700 

1 800 

900 


61 

14-283 

24-58 

81-69 

37-47 

42-47 

46-95 

51-03 

54-81 

58-34 

61-68 

51 

52 

14-422 

24-66 

31-75 

37-52 

42-52 

46-99 

51-07 

54-85 

58-38 

61-71 

52 

63 

14-560 

24-74 

81-81 

37-58 

42-57 

47-03 

51-11 

54-88 

58-41 

61-74 

53 

54 

14-697 

24-82 

31-87 

37-63 

42-61 

47-07 

51-15 

54-92 

68-45 

61-77 

54 

55 

14-832 

24-90 

31-94 

37-68 

42-66 

47-12 

51-19 

54-95 

58-48 

61-81 

55 

56 

14-967 

24-98 

32-00 

37-74 

42-71 

47-16 

51-22 

54-99 

68-51 

61-84 

56 

67 

15-100 

25-06 

32-06 

37-79 

42-76 

47-20 

51-26 

55-03 

58-55 

61-87 

57 

58 

15-232 

25-14 

32-12 

37-84 

42-80 

47-24 

51-30 

55 06 

58-58 

61-90 

58 

59 

15-362 

25-22 

82-19 

37-89 

42-85 

47-29 

51-34 

55-10 

58-62 

61-94 

59 

60 

15-492 

25-30 

82-25 

37-95 

42-90 

47-33 

51-38 

55-14 

58-65 

61-97 

60 

61 

15-620 

25-38 

32-31 

38-00 

42-94 

47-37 

51-42 

55-17 

58-69 

62-00 

61 

62 

15-748 

25-46 

32-37 

38-05 

42-99 

47-41 

51-46 

55-21 

58-72 

62-03 

62 

63 

15-875 

25-53 

32-43 

38-11 

43-03 

47-46 

51-50 

55-24 

58-75 

62 06 

63 

64 

16-000 

25-61 

32-50 

38-16 

43-08 

47-50 

51-54 

55-28 

58-79 

62-10 

64 

65 

16-125 

25-69 

32-56 

38-21 

43-13 

47-54 

51-58 

55-32 

68-82 

62-13 

65 

66 

16-248 

25-77 

32-62 

38-26 

43-17 

47-58 

51-61 

55-35 

58-86 

62-16 

66 

67 

16-371 

25-85 

32-68 

38-31 

43-22 

47-62 

51-65 

55-39 

58-89 

62-19 

67 

68 

16-492 

25-92 

32-74 

38-37 

43-27 

47-67 

51-69 

55 43 

58-92 

62 23 

68 

69 

16-613 

26-00 

32-80 

38-42 

43-31 

47-71 

51-73 

55 46 

58-96 

62 26 

69 

70 

16-783 

26-08 

32-86 

38-47 

43-36 

47-75 

51-77 

55-50 

58-99 

62-29 

70 

71 

16-852 

26-15 

32-92 

38-52 

43-41 

47-79 

51-81 

55-53 

59-03 

62-32 

71 

72 

16-971 

26-23 

32-98 

38-57 

43-45 • 

47-83 

51-85 

55 57 

59 06 

62 35 

72 

73 

17-088 

26-31 

33-05 

38-63 

43-50 

47-87 

51-88 

55-61 

59-09 

62-39 

73 

74 

17-205 

26-38 

33-11 

38-68 

43-54 

47-92 

51-92 

55-64 

59-13 

62-42 

74 

75 

17-321 

26-46 

33-17 

88-73 

43-59 

47-96 

51-96 

55 68 

59-16 

02-45 

75 

76 

17-436 

26-53 

33-23 

38-78 

43-63 

48-00 

52-00 

55-71 

59-19 

62-48 

76 

77 

17-550 

26-61 

33-29 

38-83 

43-68 

48-04 

52-04 

55-75 

69-23 

62 51 

77 

78 

17-664 

26-68 

33-35 

38-88 

43-73 

48-08 

52-08 

55-79 

59-26 

62-55 

78 

79 

17-776 

26-76 

33-41 

38-94 

43-77 

48-12 

52-12 

65-82 

59-30 

62-58 

79 

80 

17-889 

26-83 

33-47 

38-99 

43-82 

48-17 

52-15 

55-86 

59-33 

62-61 

80 

81 

18-000 

26-91 

33-53 

39-04 

43-86 

48-21 

52-19 

55-89 

69-36 

62-64 

81 

82 

18-111 

26-98 

33-59 

39-09 

43-91 

48-25 

52-23 

55 93 

69-.40 

62-67 

82 

83 

18-221 

27-06 

33-65 

39-14 

43-95 

48-29 

52-27 

55-96 

69-43 

62 71 

83 

84 

18-830 

27 13 

33-70 

39-19 

44-00 

48 33 

52-31 

56 00 

59-46 

62-74 

84 

85 

18-439 

27-20 

33-76 

39-24 

44-05 

48-37 

52-35 

66 04 

59-50 

62-77 

85 

86 

18-547 

27-28 

83-82 

39-29 

44-09 

48-41 

52-38 

56-07 

59-53 

62-80 

86 

87 

18-655 

27-35 

33-88 

89-34 

44-14 

48-46 

52-42 

56-11 

59-57 

62-83 

87 

88 

18-762 

27-42 

33-94 

39-40 

44-18 

48-50 

52-46 

56-14 

59-60 

62 86 

88 

89 

18-868 

27-50 

34-00 

39-45 

44-23 

48-54 

52-50 

56-18 

69 63 

62-90 

89 

90 

18-974 

27-57 

34-06 

39-50 

44-27 

48-58 

52-54 

56-21 

69-67 

62-93 

90 

91 

19-079 

27-64 

34-12 

39-55 

44-32 

48-62 

52-57 

66-25 

59-70 

62-96 

91 

92 

19-183 

27-71 

34-18 

39-60 

44-36 

48-66 

52-61 

66-28 

59-73 

62-99 

92 

93 

19-287 

27-78 

34-23 

39-65 

44-41 

48-70 

52-65 

56-32 

59-77 

63-02 

93 

94 

19-391 

27-86 

34-29 

39-70 

44-45 

48-74 

52-69 

66-36 

59 80 

63-06 

94 

95 

19-494 

27-93 

34-35 

39-75 

44-50 

48-79 

52-73 

56-39 

69-83 

63-09 

95 

96 

19-596 

28-00 

34-41 

39-80 

44-54 

48-83 

52-76 

66-43 

59-87 

63-12 

96 

97 

19-698 

28 07 

34-47 

89-85 

44-59 

48-87 

52-80 

66-46 

59 90 

63-15 

97 

98 

19-799 

28-14 

34-53 

39-90 

44-68 

48-91 

52-84 

66-60 

69-93 

63-18 

98 

99 

19-900 

28-21 

34-58 

89-95 

44-63 

48-95 

52-88 

56-53 

59-97 

63-21 

99 

100 

20-000 

28-28 

34-64 

40-00 

44-72 

48-99 

52-92 

56-67 

60-00 

63-25 

100 




































































































































Test No. 


INDEX 


A 


Page 


Absolute O.G.S, Units, Table relating Practical and , . 643 

Absorption Brake for measuring Horse-power of Motors . 234 & 621 

-Dynamometer . . . . . .633 

-Dynamometer, Cradle ..... 621 

23 -of Light by Shades and Globes, Measurement of the . 68 

Air at different Temperatures and Pressures, Table of Densities 

of Dry . . . . . . . .650 

-Condenser, Kelvin’s . . . . . .616 

119 -Gap in Closed Magnetic Circuit on Impedance, Keactance, 

Self-Induction, Current and Power, Effect of Length of . 341 

79 -Gaps in a Dynamo, Effect on the Magnetic Characteristic of the 211 

Alternating and D.C. Electro-Motors, General Eemarks on the 

Testing of . . . . . . .232 

127 --Current, Absolute Measurement of Self-Induction by Row¬ 

land's Method and ...... 363 

■-Current Circuits, Effects of Variation of Frequency in . 310 

117 -Current Circuits, Load and Wattless Currents in Inductive 336 

109 -Current Circuits, Measurement of Power Factor in ,. . 316 

135 -Current Circuits, Measurement of Power in 3-phase 388 & 395 

-Current Circuits, Measurement of Power in 2-phase 393 & 395 

133 --Current Circuits, Power absorbed in, by 3-Voltmeter method 379 

134 -Current Circuits, Power (by 3-Ammeter Method) in . . 383 

-Current Circuits, Relation of Supply Factors to Constants of 310 

105 -Current Commutator Motors, Efficiency and B. H. P. of Single¬ 

phase ........ 299 

128 -Current, comparison of two Self-Inductions by Rowland’s 

Method and . ...... 365 

100 -Current Electro-Motors, Efficiency and B.H.P. of Multi-phase 272 

99 -Current Electro-Motors, Efficiency and B.H.P. of Single- 

phase ........ 269 

124 -Current Frequency, Method of Separating Iron Losses . 352 

-Current, General Remarks on Measurement of Self-Induction 

by Rowland’s Method and ..... 362 

-Current Induction Motors, Testing of Asynchronous . 260 

126 -Current, Measurement of Self-Induction by using Single-phase 358 

146 & 149- Current Rotatory Converters, Efficiency of Multi¬ 
phase ....... 433 & 439 

136-143-Current Static Transformers, Measurement of Efficiency 

of . . . • * ♦ • 400—426 

106 -Current Synchronous Motors, Excitation and Armature Cur¬ 

rent and V Curves of . . . . . . 305 

66 Alternator (at Constant Excitation), Variation of E.M.F. with 

Speed in an • . . • • • • 168 


667 





























668 


INDEX 


Test No. 

67 Alternator (at Constant Speed), Variation of E.M.F. ■with Exci¬ 
tation in an . . . . • • • 

71 -Characteristic for different Po'wer Factors 

70 -Determination of the Characteristic of an 

154 -Determination of the Periodic E.M.F. and Current Curves 

of an ....«•** 
— Determination of the Periodic E.M.F. and Current Curves 


155 

73 

68 

67 

85 

72 

69 

133 

74 
126 


of an •....*** 

— Efficiency without loading it up . 

— Graphical Deduction of Total Characteristic of an . 

— Internal Losses in an . . . . • 

— Iron Losses by Retardation Method .... 
—• Magnetization Curve on Full Load of an , 

— Magnetization or Open Circuit Characteristic of an . 

- Measurement by Transmission Dynamometer of Efficiency 

of an ....... • 

■ -Relation between Exciting and Armature Currents for Con¬ 

stant Voltage on Load ..... 

-Separation of the Internal Losses in an . . 

■ -Short Circuit Characteristic of an . 

-“ Voltage Drop ” in an . . - . . 

Alternators, 3-voltmeter Method of finding the Electrical Power 
developed by . 

-Efficiency and External Loss Test of a Pair of 

■ -Impedance, Reactance, Self-Induction by Alternating 

Currents . . . . . . . 


Page 

169 

181 

177 

446 

450 

185 
179 

186 
190 
171 
169 

230 

184 

189 

173 

174 

379 

192 

358 

645 


2& 3 
4 

1 

13 

6 

77 


Aluminium and Copper, Comparative Table for . 

Ammeter Calibration by comparison with a Crompton Potentio¬ 
meter . . . . . . . .11 

-Calibration by comparison with a Kelvin Composite Balance 7 & 8 

Calibration by comparison with a Kelvin Centi-Ampere 


26 

31 

24 

25 
30 
28 

29 


Balance . . . . . . .10 

-Calibration by comparison with Standard D’Arsonval Am¬ 
meter . . . . . . . . 5 

-Complete Test for Various Sources of Error in Direct and 

Alternating Current . . . . . .29 

-Standardization by means of a Copper Voltameter . . 14 

Amsler’s Planimeter, Direction for using to give area of Indicator 

Diagram ....... 530 

-Planimeter, Instructions for working . . . 528 

Analysis of Total Internal Loss of Power in Dynamos and Motors 202 
Angle of Lead of the brushes of a Generator . . .197 

Anti-inductive Resistance for use with Kelvin Standard Electric 

Balance ....... 553 

Anti-logarithms, Tables of ..... 659 

Arc Lamp Photometer Cradle ..... 587 

Arc Lamps, Determination of the Distribution of Light from . 67 

-Examination of Alternating Current . . . .74 

•-General Remarks on the Photometry of Electric . . 61 

-Measurement of the Commercial Efficiency of . .63 

-Measurement of the Nett Optical Efficiency of . .66 

-Relation between Voltage and Current in . . .72 

-Relation between Voltage and Current with consumption of 

Carbons in . . . . . . .70 

-Relation between Voltage and Distance between the Carbons in 71 


































INDEX 


669 


Test Ko. 
35 
37 


Armature Resistance of Dynamos and Motors, Measurement of the 

- Resistance of Dynamos and Motors (Simple Potentiometer 

Method) 

Asynchronous A.C, Induction Motors, General remarks on 
Atomic Weights, Table of . 


Page 

84 

88 

260 

644 


132 


48 

105 

107 

142 

153 


93 

89 
92 
95 

100 

94 

99 

90 


B 

Balance, Kelvin Standard Centi-Ampere .... 

-Kelvin Standard Composite ..... 

Ballistic Method of measuring Electrostatic Capacity of Con¬ 
centric Cables ....... 

Bar Photometer, Trotter’s Direct Reading 

Batteries, Measurement of the Insulation Resistance of Storage . 
B.H.P. and Efficiency, etc., of Single-phase Commutator Motors 

-and Efficiency, etc., of Synchronous A.C. Motors 

Blakesley’s 3-Dynamometer Method of measuring Transformer 
Efficiency ....... 

Booster, Efficiency of a Motor-Generator or . . . 

Brake Absorption Dynamometer or Prony 

-Eddy Current ...... 

-for measuring H.P. of Motors . . . . 

-H.P. and Efficiency of D.C. Compound Wound Electro- 

Motors . 

- H.P. and Efficiency of D.C. Series Wound Electro-Motors .' 

- H.P. and Efficiency of D.C. Shunt Wound Electro-Motors . 

- H.P. and Efficiency of Electro-Motors (Swinburne’s Electrical 

^lethod) ..... . . 

- H.P. and Efficiency of Multi-phase A.C. Electro-Motors 

- H.P. and Efficiency of Small Electro-Motors (Cradle Balance 

IMethod) • ...... 

- H.P. and Efficiency of Single-phase A.C. Electro-Motors . 
H.P. and Efficiency of 500 Volt Direct Current Tram and 


Railway Motors 
- Pulley, Maw’s rule for finding the smallest size of a . 
Soames’ Motor-testing 


5i6 

554 

377 

589 

127 

299 

309 

422 

444 

633 

635 

234 

251 
241 
248 

255 

272 

252 
269 

245 

235 
637 

81 

625 


34 ' Bridge, Measurement of Resistance by the Wheatstone . 

-Siemens Low Resistance ..... 

43-4 Bridge-Megwer Testing Sets for Insulation and Conductor R^jsist- 

Ince, Evershed’s .... 116, 117 & 541 

Britannia Joint in bare overhead wires . . . .478 

88 Brush Position on Commutator of Motor Variation of Voltage, 

Current, and Speed with ..... 239 

Bunsen’s Photometer Screen ..... 592 


41 

43 

42 
160 


C 

Cables and Wires, General Remarks on Electrostatic Capacity of 
_and Wires, &c., Insulation Resistance of, by Direct 

Deflection . . • . * - , V. v ‘j c ^ * 

_and Wires, &c.. Insulation Resistance of, by Evershed bet . 

_.and Wires, &c.. Insulation Resistance of, by Silvertown Set 

_Detailed Instructions for Jointing Electric Light . 

__General Observations on Jointing Electric Light 

_in Testinf^ Insulation Resistance, Preparation of the Ends of 


369 

99 

113 

105 

474 

473 

99 

























670 


INDEX 


Test No. Page 

131 Cables, Measurement by Kelvin Voltmeter Method of Electrostatic 

Capacity of . . . . . . . 374 

35 -Measurement of the Kesistance of Electric Light . . 84 

12 Calibration of a High Tension Alternating Current Voltmeter 26 

17 -of a High Tension Wattmeter by application of Ohm’s Law 41 

11 -of a Voltmeter by comparison with a Crompton Potentio¬ 
meter . • . . . . . . .23 

10 -of a Voltmeter by comparison with a Kelvin Ceuti-Ampere 

Balance . . . . . . .22 

9 --of a Voltmeter by comparison with a Kelvin Composite 

. Balance . . . . . . .21 

8 -of a Voltmeter by comparison with a Standard D’Arsonval 

Voltmeter . . . . . . .18 

14 -- of a Wattmeter by comparison with a Kelvin Composite 

Balance ....... 34 

15 -- of a Wattmeter by comparison with a Standard Ammeter 

and Voltmeter . . . . . .37 

16 -of a Wattmeter with Alternating Currents (3-Voltmeter 

Method ....... 38 

4 -of an Ammeter by comparison with a Kelvin Centi-Ampere 

Balance .... ... 10 

2 -of an Ammeter by comparison with a Kelvin Composite 

Balance ....... 7 

3 -of an Ammeter by comparison with a Kelvin Composite 

Balance ....... 8 

1 -of an Ammeter by comparison with a Standard D’Arsonval 

Ammeter . . . . . . .5 

5 -of an Ammeter by means of a Crompton Potentiometer . 11 

18 -of an Electricity Meter . . . . .43 

7 -of Direct Current Voltmeters (Poggendorff’s Method) . 16 

-of Electrical Measuring Instruments, General Remarks on the 4 

51 -of Speed Indicators ...... 134 

21 Candle Power and Efficiency of Electric Glow Lamps, Measure¬ 

ment of the ....... 50 

-Power (mean spherical) of an Electric Arc Lamp . . 70 

-Power (mean spherical) of an Electric Glow Lamp . . 56 

22 -Power, Variation of, with direction around an Electric Glow 

Lamp ........ 55 

113 Capacity and Ohmic Resistance in Circuits, Effect of Frequency 

with ........ 326 

33 -and Efficiency of Secondary Cells, Measurement of the Storage 76 

■ -and Efficiency of Secondary Cells, Ways of Denoting . 78 

-Current of a Cable or Main ..... 372 

-of Cables (Kelvin Voltmeter Method), Proof of Formula in . 500 

■ -of Cables (Magneto Inductor Method), Proof of Formula in 501 

132 -of Concentric Cables (ballistically), the Electrostatic . 377 

-of Concentric Cables, Formula expressing the . . 371 

130 -of Concentric or Ordinary Cables and Condensers (A.C. 

Method), Electrostatic ..... 372 

-of Electrical Wires and Cables, Measurement of the Electro¬ 
static .... ... 372-9 

131 -of Short Cables (Kelvin “ Voltmeter Method ”), the Electro¬ 

static ...... . 874 

Carbon Rheostat, Adjustable ..... 596 

Carhart-Clark Standard Cell, Temperature coefficient of the 17 & 643 

































INDEX 


671 


62 


63 


Test No. 

Cells, Proof of Formula giving Internal Resistance of Secondary 
Centi-Ampere Balance, Constants (when used as a Voltmeter) of 
Kelvin’s ....... 

-Ampere Balance, Kelvin’s ..... 

“ Characteristic ” of a Compound Wound Dynamo (Long Shunt), 
Determination of the ...... 

-of a Compound Wound Dynamo (Long Shunt), Graphical 

deduction of Total ...... 

-of a Compound Wound Dynamo (Short Shunt), Determina¬ 
tion of the ....... 

--- of a Compound Wound Dynamo (Short Shunt), Graphical 

deduction of Total ...... 

;-of a Dynamo with varying Air Gaps, Magnetic 

-- of a Rotary Converter, “ No Load ” (open circuit) . 

-of a Shunt Wound Dynamo, Determination of the . 

■-- of a Shunt Wound Dynamo, Graphical Deduction of Total . 

-- of an Alternating Current Generator, Determination of 

-of an Alternator, for Different Power Factors 

-of an Alternator, Graphical Deduction of the Total . 

--- of an Alternator, The Short Circuit . . . . 

-of Magneto Dynamo, Graphical Determination of Total 

•-of Separately Excited Dynamo, Determination of the 

-of Series Wound Dynamo, Determination of the External . 

-of Series Wound Dynamo, Graphical Deduction of Total 

-of Static Transformers, Magnetization or Open Circuit , 

and Efficiency of Rotary Converters (Run from D. C. 


79 ;- 

147 - 

GO - 

70 - 

71 -- 

69 -- 

55 -- 

58 - 

136 - 

146&149 


Page 

491 

550 

546 

160 

162 

163 

164 
211 
438 
155 
157 
177 
181 

- 179 
173 
145 
147 
150 
152 
404 


56 


59 


61 


54 


118 


116 


113 


112 


110 


115 


111 


and A.C. Sides 

-or Curve of Magnetization of Separately Excited 

Internal ..... 

- or Curve of Magnetization of Series Wound 

Internal . . 

-or Curve of Magnetization of Shunt Wound 

Internal ..... 

“Characteristics ” of Dynamos, Introductory Remarks on the 
of Magneto Dynamo, Determination of the . 


433 & 439 
Dynamo, 


Dynamo, 

• • 

Dynamo, 


Chemical Equivalents, Table of 
Choker, Variation of Impedance, Reactance and Self-Induction 
with Position of Movable Core in Solenoidal . 

Circuits (A.C.)> General Remarks on Constants and Supply of 

-containing Capacity and Self-Induction in Parallel, Variation 

of Impedance and Phase Relations between Currents in 

--- Effects of Variation of Frequency of Supply in Alternating 

Current . . . • • • 

-having Capacity and Resistance only. Variation of Imped¬ 
ance with Capacity, Frequency and Resistance 

-having Capacity in Series with Resistance, Numerical and 

Phase Relation between Voltages and between Voltage and 
Current . . . • • • , • 

-having Ohmic Resistance and Self-Induction only. Relation 

between Frequency and Temperature . . . 

-having Resistance in Parallel with Self-Induction or Capacity, 

Numerical and Phase Relation between Main and Branch 

Currents . . • • • . 

-having Self-Induction and Resistance only. Variation of 

Impedance with Self-induction, Frequency and Resistance . 


148 

155 

159 

142 

143 
644 


339 

310 

333 

310 

326 


323 


319 


332 

321 































672 


INDEX 


Test No. Page 

114 Circuits having Self-Induction, Capacity and Resistance, Variation 
of Impedance with Self-Induction, Capacity, Resistance and 
Frequency ....... 328 

-Measurement of Power in 2-Phase . . . 393 & 395 

135 -Measurement of Power in 3-Phase . . . 388 & 395 

Clark Standard Cell, Preparation of the .... 504 

Clark’s Standard Cell, relation between E.M.F. and tempera¬ 
ture of . . . . . . 17 & 643 

Coefficient of Magnetic Leakage in Machines (Ballistic Method) . 207 

of Magnetic Dispersion in Induction Motors . 


78 

128 

24 

73 

85 

84 

105 

64 

65 
62 


53 

63 


93 

132 

130 


288 

365 

63 

185 

230 


Coefficients of Self-Induction (by Alternating Currents), Com¬ 
parison of two ...... 

Commercial Efficiency of an Arc Lamp, Measurement of the 

-Efficiency of an Alternator ..... 

-Efficiency of a Generator, Determination by Transmission 

Dynamometer ....... 

-or Nett Efficiency ofD.C. Dynamos (Kapp’s Electrical Method) 226 

Commutator Motors, Efficiency and B.H.P. of Single Phase . 299 

Composite Balance, Constants for Kelvin’s . . 556-8 

-Balance, Kelvin’s .....* 554 

Compounding a Dynamo, Determination of Field Magnet Wind¬ 
ings for truly . . . . . . .165 

-a Dynamo, Determination of Speed and E.M.F. for truly . 167 

Compound Wound Dynamo (Long Shunt), Determination of the 

Characteristic of a . . . . . .160 

-Wound Dynamo (Long Shunt), Graphical deduction of Total 

Characteristic of a . . . . . .162 

-Wound Dynamo, Relation between Speed and E.M.F. in . 140 

-Wound Dynamo (Short Shunt), Determination of the 

Characteristic of a . . . . . .163 

-Wound Dynamo (Short Shunt), Graphical deduction of Total 

Characteristic of a . . . . . .164 

-;Wound Electro-Motors, Efficiency and B.H.P. of Direct 

Current ....... 251 

Concentric Cables, Measurement ballistically of Electrostatic 

Capacity of . . . . . . . 377 

-Main, Straight Joint in a . . . . . 486 

-or Ordinary Cables and Condensers (A.C. Method), Electro¬ 
static Capacity of . . . . . .372 

Condensers, Kelvin’s Air ...... 616 

Conductivity Test of Copper Wire by Siemens’ Low Resistance 

Bridge. . . . . . . . 527 

38 & 44 Conductor Resistance, Measurement by a Portable Testing Set 

of Metallic ...... 90 & 117 

6 “ Constant ” of an Ammeter, Determination of, by a Copper 

Voltameter . , . . . . .14 

Constants, useful ....... 654 

Contact Maker, Revolving ...... 619 

Continuous and A.C. Electro-Motors, General Remarks on the 

Testing of ...... . 232 

Conversion in Multi-phase Rotaries, Voltage ratio of , 437 & 439 

-ratio of A.C. Static Transformers .... 402 

146 & 149 Converters, Efficiency of Multi-phase Rotatory . 433 & 439 

147 -“ No Load ” characteristic of Rotary . . . 438 

148-152-Other tests on Rotary .... 439-443 



















INDEX 


673 


Test No. 

Converters, Synchroniring Eotary . 

Copper and Aluminium, Comparative Table for . 

-Conductors, Standards for . 

-Electro-Chemical Equivalent of 

137 -Losses in Transformer (Short Circuit Test) . 

Voltameters, Directions as to the use and arrangement of 


94 


Page 
. 441 

. 645 

640-2 
. 14 

. 407 
. 14 

. 621 
587 


75 


76 

80 


154 


Cradle, Absorption Dynamometer 
-Arc Lamp Photometer 

-Balance Method of finding the Efficiency and B.H.P. of D.C. 

or A.C. Motors ...... 252 

Critical Resistance at a given Speed for a Series Wound Dynamo 154 

-Resistance at a given Speed for a Shunt Wound Dynamo . 159 

Commutator of a Dynamo, General Remarks on, and Thompson’s 

Method of finding the Distribution of Potential round the . 195 

-of a Dynamo, Mordey’s Method of Finding the Distribution 

of Potential round the. ..... 199 

•-- of a Dynamo, Mordey-Swinburne Method, ditto . . 201 

Coils, Localization of Faults in Magnetizing . . .213 

Crompton D’Arsonval Galvanometer .... 569 

Crompton’s Potentiometer . . . . . .510 

Current and E.M.F. Curves of an Alternator, the Periodic . 446 

-in Circuits having Ohmic Resistance and Self-Induction only. 

Relation of Frequency and . . . . .310 

Curve Plotting ....... 1 

-Plotting, Notation for points in . . ‘ .3 

-Tracer, Ewing’s Magnetic ..... 609 

Curves of Electrical Horse-power Developed . . . 147 

-Polar ........ 56 


D 


78 

25 

22 

75 

76 

156 

64 

65 
62 
63 
60 


D’Arsonval Galvanometer, Crompton .... 569 

Densities of Dry Air at different Temperatures and Pressures, 

Table of . . ..... 650 

Distribution of Waste Field in Dynamos, Determination of the 

Relative ....... 207 

-of Light from an Electric Arc Lamp . . . .67 

-of Light from an Electric Glow Lamp . . .55 

-of Potential round the Commutator of a Dynamo, General 

remarks on, and Thompson’s Method .... 195 
-- of Potential round the Commutator of a Dynamo (Mordey’s 


Method) ....... 

-of Potential round the Commutator of a Dynamo (Mordey- 

Swinburne) ....... 

Double Square Roots for Kelvin Balances, Table of 

Duddell Oscillograph, “ Wave-forms” by the 

Dynamo, Determination of Separate Field Magnet Winding for 
Truly Compounding . . . ... 

_Determination of Speed and E.M.F. which produces a True 

Compounding . . . . - i' -.tt ; 

_Determination of the Characteristic of a Compound Wound 

(Long Shunt) . . . • * ■,* -nr j 

_Determination of the Characteristic of a Compound Wound 

(Short Shunt) . . * . . V m \ w 'a 

_Determination of the Characteristic of a Shunt Wound 


199 

201 

665 

451 

165 

167 

160 

163 

155 


X X 
























674 


INDEX 


Test No. rage 

Dynamo, Determination of the Critical Kesistance (at given speed) 

for a Series ....... 154 

59 --Determination of the Curve of Magnetization of a Series 

Wound........ 155 

61 -Determination of the Curve of Magnetization of a Shunt 

Wound . . . . . . .159 

58 -Determination of the External Characteristic of a Series . 150 

-Determination of the Total Characteristic of a Compound 

Wound (Long Shunt) . . . . . .162 

-Determination of the Total Characteristic of a Comj>ound 

Wound (Short Shunt) . . . . . .164 

-Determination of Total Characteristic of Series Wound . 152 

-Determination of the Total Characteristic of a Shunt Wound 157 

-Dispersion Coefficient in Induction Motors . . . 288 

75 -Mordey’s Method of finding the distribution of Potential 

round the Commutator of a ..... 199 

76 -Mordey-Swinburne Method, ditto .... 201 

■-Thompson’s Method of finding the Distribution of Potential 

round the Commutator of a . . . . . 195 

79 -with varying Air Gaps, Magnetic Characteristic of . . 211 

Dynamometer, Absorption . . . . . .633 

-Cradle Absorption ...... 621 

85 -l\Ieasurement of Efficiency of a Geneiator by Transmission . 230 

• -Measuring Instruments, Parr’s Direct-reading . . 572 

-Siemens Electro- ...... 577 

• -Siemens Horse-poaver Transmission .... 623 

--Spring Transmission ...... 625 

-Wattmeter, Siemens ...... 580 

77 Dynamos and Motors, Analysis of Internal Loss of Power in . 202 

43 & 49-Measurement of the Insulation Resistance of . 113 & 129 

54 -Determination of the “ Characteristic ” of Magneto ' . 143 

55 -Determination of the “ Characteristic ” of separately excited 147 

83 -Efficiency by Hopkinson’s Electrical Method of Direct 

Current ....... 223 

84 --Efficiency by Kapp’s Electrical Method of Direct Current . 226 

82 —— Efficiency by Swinburne’s Electrical Method of Direct 

Current ....... 219 

56 -Internal Characteristic or Curve of Magnetization of sepa¬ 

rately excited . . . . . . .148 

-Introductory Remarks on the “ Characteristics ” of . . 142 

57 —— Relation between External and Exciting Currents of separ¬ 

ately excited ....... 149 

53 -Relation between Speed and E.M.F. in Direct Current . 140 

E 

Eddy Current Absorption Dynamometer Brake . . . 635 

123 -Currents in Magnetic Material, Measurement of . 348 & 352 

93 Efficiency and B.H.P. of D.C. Compound Wound Electro-Motors 251 

89 -and B. H.P. of D.C. Series Wound Electro-Motors . . 241 

92 -and B.H.P. of D.C. Shunt Wound Electro-Motors . . 248 

95 -and B. H. P. of Electro-Motors (Swinburne’s Electrical Method) 255 

90 -and B.H.P. of 500 Volt Direct Current Tram and Railway 

Motors........ 245 

94 -and B.H.P. of small Electro-Motors (Cradle Balance Method) 252 






































INDEX 


676 


Test No. 

100 Efficiency and B.H.P. of Multi-phase A.C. Electro-Motors 

99 -and B.H.P. of Single-phase A.C. Electro-Motors 

24 -and C. P. of Electric Arc Lamps, Measurement of 


the 


25 

21 

74 

33 


105 

107 

153 

158 
85 

144 

145 

159 
73 

157 

96 

83 

84 
82 


101 - 

102 - 

146 & 149 

143 - 

139-142 - 
142 - 


140 


139 


141 


103 

18 

19 


Commercial . . 

■andC. P. of Electric Arc Lamps, Measurement of the Nett 
Optical . . 

- and C. P. of Electric Glow Lamps, Measurement of the 
■ and Internal Loss Test of a Pair of Alternators 

- and Storage Capacity of Secondary Cells, Measurement of the 

- and Storage Capacity of Secondary Ccdls, Ways of represent¬ 
ing the . , . . , 

- B.H.P. &c., of Single-phase Commutator Motors , • 

-B.H.P. &c., of Synchronous A.C. Motors 

- of a Booster or Motor Generator Set ...» 

- of a Gas-Engine Dynamo Generating Set, the Commercial . 

- of a Generator, Measurement by Transmission Dynamometer 

of the . • . . . . ' . • 

- of a Nodon Valve Rectifier ..... 

- of a Rotary Rectifier ...... 

- of a Steam-Engine Dynamo Generating Set, the Commercial 

- of an Alternator without loading it up . . . 

- of an Electro-Motor-Fan Set . 

- of D.C. Electro-Motors (Poole’s Electrical Method) - 

- of Direct-Current Dynamos (Hopkiuson’s Electrical Method) 

- of Direct-Current Dynamos (Kapp’s Electrical Method) 

- of Direct-Current Dynamos (Swinburne’s Electrical Method) 

- of Dynamos (Hopkinson’s Method), Proof of Fommla for the 

- of Induction Motor (Heyland Method) . , 

- of Induction Motor (Sumpner-Weekes method) 

-of Multi-phase A.C. Rotary Converters . 

- of Multi-phase A.C. Static Transformers 
—— of Single-phase A.C. Static Transformers. 

- of Single-phase A.C. Static Transformers 

Dynamometer Method) . . . . . 

of Single-phase A.C. Static Transformers (by Double Con¬ 


version) 
of Single-phase A.C. 


Static Transformers (by 


144 

94 

95 


version) 

-of Single-phase A.C. Static Transformers (by Sumpner’s 

Method) ....... 

•-of Transformers (Blakesley’s Method), Proof of Formula for 

the . . . • • • * . 1 

—^—Slip, Torque, Load, &c., in Induction Motor with Variable 

Rotor Circuit Resistance, Relation between . 

Electricity Meter, Calibration of an . . . 

-Meter, Complete Test of an . 

Electro-Chemical Equivalents of Copper for various current den¬ 
sities, Table of. 

-Equivalents, Table of . . . • • 

Electro-Dynamometer, Siemens ..... 
Electrolytic Rectifier, Efficiency of a Nodon Valve . . 

Electro-Motors, Efficiency and B. H.P. (by Cradle Balance Method) 

of small . . • * . 1 Vr ,1 j*\ 

-Efficiency and B.H.P. (by Swinburne s Electrical Method) 

of ..♦•••• • 

X X 2 


Page 
272 
269 

63 

66 
50 
192 
76 

78 
299 
309 
444 
465 

230 
427 
431 
468 
185 
462 
258 
223 
226 
219 
497 
277 
. 288 
. 433 & 439 

. 424 

412-424 
(Blakesley’s 

. 422 

414 

412 

417 

502 

294 
43 
45 

41 
644 
577 
427 

252 

255 


Single 


Con- 







































G76 


INDEX 


67 


66 


125 


87 


58 


Test No. 

93 Electro-Motors, Efficiency and B.H.P. of D.C. Compound Wound 

89 -Efficiency and B.H.P. of D.C. Series Wound 

92 -Efficiency and B.H.P. of D.C. Shunt Wound 

100 -Efficiency and B.H.P. of Multi-phase A. C. . 

99 —^—■ Efficiency and B. H P. of Single-phase AC.. 

96 -(Poole’s Electrical Method), Efficiency of D.C. 

-General remarks on the Testing of continuous and A. C. 

132 Electrostatic Capacity of Concentric Cables by Standard Magneto 
Inductor Method ...... 

- Capacity of Concentric or Ordinary Cables and Condensers 
by the Alternating Current Method .... 

Capacity of Electric Wires and Cables, General Notes on 


130 


131 


155 


154 


65 


53 


the 

-Capacity of Short Cables by Kelvin Multicellular Voltmeter 

Method ....... 

-Voltmeters ....... 

-Voltmeters, Kelvin’s Multicellular .... 

E.M.F. and Current Curves of an Alternator (Ballistically), the 
Periodic ....... 

-and Current Curves of an Alternator (Electrostatically), the 

Periodic ....... 

-and Speed at which a Dynamo truly Compounds, Determina¬ 
tion of. 

-and Speed in Direct-Current Dynamos, Relation between . 

-and Temperature of Clark’s Weston Cadmium and Carhart- 

Clark’s Standard Cells, Table giving .... 

- of an Alternator, Algebraical Relation expressing the 

Total ........ 


-- of an Armature Coil at different positions (Thompson’s 

Method), Investigation of . 

-^ of Armature Coils, Thompson’s Method of Measuring the . 

-with Excitation (at constant speed) in Alternators, Varia¬ 
tion of........ 

-with Speed (at constant excitation) in Alternators, Variation 

of ....... . 

Errors in Ammeters and Voltmeters, Enumeration of • 

Eureka Resistance Material, Table for . , , 

Evershed Bridge-Megger Constant Pressure Generator for 
-Bridge-Megger Index, Adjuster for . 

■-Bridge-Megger Insulation Testing Set, Description of the . 

-Megger Insulation Testing Set, Description of the . 

-Portable Testing Sets, Description of tlie 

Ewing’s Hysteresis Tester, Measurement of Magnetic Hysteresis by 

-Magnetic Curve Tracer ..... 

Excitation with Speed of D.C, Motors, Variation of (for Constant 
Voltage) ....... 

External CliaracCeristic of Series Dynamo, Determination of the . 


Pag® 

251 

241 

248 

272 

269 

258 

232 

377 

372 

369 

374 

562 

563 

450 

446 

167 
140 

643 

179 

198 

198 

169 

168 
29 

646 

544 

545 
541 
539 
539 
356 
609 

238 

150 


F 


Factor in Induction Motors, Leakage .... 288 

109 -in A.C. Circuits, Measurement of Power . . . 316 

157 Fan Set, Efficiency of an Electro-Motor .... 462 

50 Faults in Electric Mains, Localization of . . . . 130 

80 Faults in Magnetizing Coils, Localization of . . .213 































INDEX 


677 


Test No. Page 

64 Field Magnet Windings for Truly Compounding a Dynamo, 

Determination of . . . . . .165 

Frequency, Slip and Speed, Measurement by Stroboscopic Method 262 
111 and Current, Power, Impedance, and Angle of Lag in 

Circuits having Ohmic Resistance and Self-Induction only, 
Relation between ...... 321 

110 and Temperature in Circuitshaving Ohmic Resistance and 

Self-Induction only, Relation between . . .319 

113 -on Circuits having Capacity and Ohmic Resistance, Effect of 326 

114 -on Circuits having Self-Induction, Capacity, and Ohmic 

Resistance, Effect of . . . . . . 328 

-of the Supply in A.C. Circuits, Effects of Variation of the . 310 

Fuse Table for Different Diameters and Currents . , , 654 

G 

Galvanometer, Crompton D’Arsonval .... 569 

Galvanometers, Deviation of deflection from direct proportionality 

in Reflecting ....... 490 

-Sensitive Portable . . .... 571 

158 Gas-Engine Generating Set, the Commercial Efficiency of. . 465 

Gauges, Weights, Resistances and Sections, Table giving relation 

of different metals of . . . . . . 653 

-Comparison of different Wire ..... 650 

Gearing of Engine Indicators ..... 469 

Generator, the Evershed Bridge-Megger Constant Pressure . 544 

21 Glow Lamp, Measurement of the Efficiency and C.P. of Electric . 50 

22 -Variation of C.P. with direction around an Electric . . 55 

Guard-wire in Tests on the Insulation Resistance of Cables, Price’s 100 

H 


101 Heyland Diagram, Experimental and Graphical Deduction of . 277 

74 Hopkinson Principle for Testing a Pair of Alternators . . 192 

83 Hopkinson’s Electrical Method of Measuring Efficiency of D.C. 

Dynamos ....... 223 

-Method of Measuring Dynamo Efficiency, Proof of Formula 497 

122 -^ Permeameter, Measurement of Magnetic Permeability by . 347 

102 Hopkinson-Surapner Method of Testing Induction Motors . 288 

Horse-power Curves ...... 147 

-Transmission Dynamometer, Siemens . . . 623 

Houseman’s Method of Separating the Internal Losses in a Dynamo 

or Motor ....... 204 

125 Hysteresis by Ewing’s Hysteresis Tester, Measurement of Magnetic 356 


Magnetic ....... 348 

-Test, Preparation of Iron samples for . . . 350 

Horizontal Candle Power, Mean . . . . .56 


I.H.P. of a Compound or Trii3le-Expansion Engine, Determination 

of the ,...•••• 473 

Illumination Photometer, Trotter’s .... 590 
















678 


INDEX 


Test No. 

116 Impedance and Phase Relations between Currents in Circuits 
having Capacity and Self-Ind. in Parallel, Variation of 

118 -, Reactance and Self-Ind. with Position of Movable Core in 

Solenoidal Choker ...... 

126 -, Reactance, and Self-Ind. of Alternators, Motors, Trans¬ 

formers, &c. by Alternating Currents .... 

119 -, Reactance, Self-Ind,, Current and Power, Effect of Length 

of Air Gap in a Closed Magnetic Circuit on . 

with Capacity, Frequency, and Resistance in Circuits having 


113 


114 


111 


61 


101 

102 

104 

100 

99 

97 

98 


103 


117 


120 


108 


42 


41 


49 


47 

43 


Capacity and Resistance only, "Variation of 
- with Self-Ind., Capacity, and Resistance in Circuits having 
Self-Ind., Capacity, and Resistance, Variation of 
with Self-Ind., Frequency and Resistance in Circuits having 


Self-Ind. and Resistance only. Variation of 
Indicator Diagram, Determination of the I. H. P. from the 

-Diagram, Form and Explanation of an 

Indicators, Calibration of Speed .... 
Gearing of Engine ..... 
Table of Springs for Thompson’s Steam-Engine 
Thompson’s Steam-Engine 


Induction Motor, Complete Test without loading it up . 

Motor, Complete^Test by Sumpner-Weekes method . 

Motor, Relation between Starting Torque, Current, Voltage, 


and the Rotor Circuit resistance of an 

- Motors, Efficiency and B. H. P. of Polyphase 

- Motors, Efficiency and B.H.P. of Single Phase 

- Motors, No-Load Open-circuit Test on , 
Motors, No-Load Short-circuit, Test of 


Motors, Ratio of Transformation in 


-Motors, Testing of Asynchronous A. C. 

-Motors with variable Rotor Circuit Resistance, Relation 

between Efficiency, Slip, Torque, Load, &c., in 
Inductive A.C. Circuits, Determination of Load and "Wattless 
Currents in . 

Effects due to the relative Positions of 2 Coiled Circuits, In¬ 


vestigation of Mutual 

- Drop of an Alternator with Load and Power Factor . 

- Retardation ...... 


Page 

333 

339 

358 

341 

326 

328 

321 

471 

470 

134 

469 

534 

531 

277 

288 

296 

272 

269 

264 
267 

265 
260 

294 

336 

343 

174 

370 

314 


Inductiveness of a Circuit, Determination of the . 

Instruments, General Remarks on the Calibration of Electrical 

Measuring ....... 4 

Insulation Resistance, Actual Values for given Voltage and 

Number of Lamps ...... 112 

-Resistance by Direct Deflection Method, Proof of Formula for 493 

-Resistance, General Remarks on . . . 98 & 109 

-Resistance, Measurement by the Silvertown Portable Testing 

Set ........ 105 

-Resistance of Cables, Measurement by Direct Deflection 

Method ....... 99 

— Resistance of Dynamos and Motors, Measurement of the . 129 

— Resistance of Dynamos and Motors, Proof of Formula for . 496 

—• Resistance of Electrical Cables and Installations . 98 & 109 

— Resistance of Faulty Telegraph Insulators . . .123 

— Resistance of Installations, etc., by Evershed’s Portable 

Megger and Bridge-Megger Testing Sets , 113-116 & 639 



































INDEX 


679 


Test No. 

45 Insulation Resistance of Installations, etc., while working 

46 -Resistance of Installations, etc., while working 

-Resistance of Installations while working. Proof of Formula 

for _ . . . 

48 -Resistance of Storage Batteries, Measurement of the 

-Resistance of Storage Batteries, Proof of Formula for 

-Resistance with Testing Voltage, Variation of 

Insulators for Postal and other Telegraph Lines . 

56 Internal Characteristic of Separately Excited Dynamos . 

59 -Characteristic of Series Wound Dynamos 

61 -Characteristic of Shunt Wound Dyuamo 

74 -Loss and Efficiency Test of a Pair of Alternators 

77 -Loss of Power in Dynamos and Motors, Analysis of the 

-Losses in an Alternator . . . . . 

-Losses in an Alternator, Separation of the 

32 --Resistance of Secondary Cells, Measurement of the . 

-Resistance of Secondary Cells, Proof of Formula giving 

Iron Losses in Alternators by Retardation Method 
124 -Losses, Separation of, by A, C. Frequency Method . 


Page 

119 

121 

494 

127 

496 

113 

123 

148 

155 

159 

192 

202 

186 

189 
75 

491 

190 
352 


J 

Jointing Electric Light Cables and Mains, Course in 

160 -Electric Light Cables, Detailed Instructions for 

-Electric Light Cables, General Observations on 

Jolly’s Photometer Screen . . . 


475 

474 

473 

593 


K 


84 


131 


Lapp’s Electrical Method of Measuring Efficiency of D.C. Dynamos 

-Method of Separating the Internal Losses in Dynamos and 

Motors . #..•.* 

Kelvin Air Condenser ...... 

-Magneto-static Current Meter .... 

-Multicellular Electrostatic Voltmeter 

-Standard Balances, Adjustment of the 

-Standard Centi-Ampere Balance .... 

-Standard Centi-Ampere Balance, Anti-Inductive Resistances 

for . . . . . . . 


226 

203 

616 

558 

563 

550 

546 

553 


550 

554 


• Standard Centi-Ampere Balance, Constants when used as a 
Voltmeter ....... 

Standard Composite Balance ..... 

Standard Composite Balance, Constants for . . 556-8 

• “Voltmeter Method” of finding Electrostatic Capacity of 

... 374 

. . . 586 

. . • 583 

. 585 
. 587 


Short Cables 
Key, Highly insulated 2-way spring tapping 

-Pohl’s change-over commutator 

-Simple reversing . 

■-Simple 2-way sliding . 


L 


111 Lag in Circuits having Ohmic Resistance and Self-Induction only, 

° Effect of Frequency on Angle of • • 

Lagging and Leading Currents in a Circuit, the Production of . 


321 

181 






























680 


INDEX 


Test No. Page 

Lamp-box Resistance, Incandescent' . • . . 598 

129 Laws of Combination of Self-Inductions in Series and Parallel . 366 

Lead-covered Cables, Twist-Joint in . . . .483 

-Cables, T-Joint in ...... 484 

Lead of Brushes in a Dynamo, Angle of . . . . 197 

78 Leakage Coefficient “ ” in Dynamos, Determination of the 

^Magnetic ....... 207 

-Factor in Induction Motors ..... 288 

Liquid Rheostat, Three-phase ..... 607 

Liquids, Table of Specific Resistance of . . . . 645 

117 Load and Wattless Currents in an Inductive A C. Circuit, 

Determination of the ...... 336 

100 Load-Efficiency Test of an Induction Motor . . .272 

50 Localization of Faults in Electric Mains .... 130 

80 -of Faults in Magnetizing Coils .... 213 

Low Resistance, Approximate Test for a very . . . 524. 

-Resistance Bridge, Siemens form .... 525 

-Resistance Fixed Standard . . . , .604 

-Resistance Measurer, Description of a . . .521 

40 -Resistance, Measurement by “ Nalder ” Measurer . 97 & 521 

37 -Resistance, Measurement by Simple Potentiometer Method . 88 

36 -Resistance, Measurement by Voltmeter and Ammeter 

Method ....... 86 


35 -Resistance (Potential Difference Method), Measurement of . 84 

-Resistance (Potential Difference Method), Solution of Infer¬ 


ences for ...... . 492 

Logarithms, Tables of ..... . 657 

124 Losses in Iron, Separation of, by A.C. Frequency Method . 352 

137 -in Transformers (by Short Circuit Test), Copper . . 407 

-in Transformers on Open Circuit (constant) . . .403 

Lummer-Brodhun Photometer Screen, Princiide of . . 593 


M 

81 Magnet Coils, Rise of Temperature and Increase of Resistance of. 216 
79 Magnetic Characteristic of a Dynamo with Varying Air Gaps . 211 

-Curve Tracer, Ewing’s ..... 609 

-Dispersion in Induction Motors, Coefficient of . . 2S8 

125 -Hysteresis, by Ewing’s Hysteresis Tester, Measurement of . 356 

123 -Hysteresis in Magnetic Material (Single-phase A.C.s), 

Measurement of ..... . 348 

78 -Leakage Coefficient “y” in Dynamos, Determination (by 

Ballistic Method) of the ..... 207 


123-4 -Material, Measurement of Hysteresis (Single-phase A.C.s), 

in samples of ...... 348-352 

122 -Permeability, by Hopkinson’s Permeameter, Measurement of 347 


121 

-Permeability by the Permeameter, Measurement of . 

-Slip in A.C. Motors ..... 

. 345 

260-2 

67 

Magnetization Characteristic of an Alternator 

• » 

. 169 

68 

-Curve of an Alternator on Full Load 

• • 

. 171 

56 

-Curve of Separately Excited Dynamo 

• • 

. 148 

59 

-Curve of Series Wound Dynamo 

• • 

. 155 

61 

-Curve of Shunt Wound Dynamo 

• • 

. 159 

136 

-Curve or “Open Circuit” Characteristic 

formers .... 

of Static 

• • 

Trans- 

. 404 

































INDEX 


681 


Test No. Page 

147 Magnetization Curve or “ Open Circuit” Characteristic of Rotary 

Converter . . . . . . .438 

80 Magnetizing Coils, Localization of Faults in . . . 213 

54 Magneto Dynamo, Determination of the Characteristic of a . 143 

-Dynamo, Graphical Deduction of Total Characteristic of a . 145 

Magneto-static Current Meter, Kelvin’s adjustable . . 558 

Manganin Resistance Material, Table giving Rise of Temperature 

for Different Currents in .... . 649 

Maw’s Rule for finding the smallest size of a Brake Pulley . 235 

Mean-spherical and Horizontal Candle Power . . .56 

Measuring Instruments, Parr’s Direct-Reading Dynamometer . 572 

“ Megger” Insulation Testing Set, Evershed’s 
Meter, Calibration of an Electricity 
Complete Test of an Electricity 


18 

19 

75 


114 & 539 
. 43 

. 45 

. 595 


76 

153 

86 

88 

87 

102 

101 

94 

107 

93 

89 
92 

90 

100 

99 

105 

95 

126 

97 

98 

96 

104 

91 

106 


^Methven Screen Photometric Standard of Light . 

Mordey’s Method of finding the Distribution of Potential round a 
Dynamo Commutator ...... 

-Method of Separating the Internal Losses in Motors and 

Dynamos ....... 

Mordey-Swinburne Method of findingthe Distribution of Potential 
round a Dynamo Commutator .... 

Motor Generator Set, Efiiciency of a Booster or . 

-Testing Brake, Soames’ ..... 

Motors (at Constant Excitation), Variation of Speed with Voltage 
on Armature ....... 

-(at Constant Excitation), Variation of Speed, Voltage Current 

with Brush position on Commutator of . . . 

-(at Constant Voltage), Variation of Speed with Excitation 

in D.C. ....... 

-Complete Test by Sumpner-Weekes method . 

-Complete Test without loading up, of Induction 

-Efficiency and B.H.P. (by Cradle Balance Method) of small 

Electro- ....... 

■-Efficiency and B.H.P., etc., of Synchronous . 

-Efficiency and B.H.P. of D.C. Compound Wound Electro- . 

-Efficiency and B.H.P. of D.C. Series Wound Klectro- 

-Efficiency and B.H.P. of D.C. Shunt Wound Electro- 

-Efficiency and B.H.P. of 500-Volt D.C. Series Wound 

Tramway 

-Efficiency and B.H.P. of Multi-phase A.C. Electro- . 

_Efficiency and B.H.P. of Single-phase A.C. Electro- 

_Efficiency and B.H.P. of Single-phase Commutator . 

Efficiency and B.H.P. (Swinburne’s Electrical Method) of 


Reactance, and Self-Ind. by Alternating 


Electro- 

- Impedance, 

Currents . • • • . • 

- No-Load “ Open Circuit ” Test of Induction 

- No-Load Short-Circuit Test of Induction 

- (Poole’s Electrical Method), Efficiency of D.C. Electro- 

- Ratio of Transformation in Induction . * , , * 

Relation between Starting, Torque, Current, Voltage, and the 


Rotor Circuit Resistance of an Induction _ 

- Relation between Starting Torque and Current in D.C. 

- Relation of Excitation to Armature Current and V Curves 

of Synchronous ,..••• 


199 

203 

201 

444 

637 

236 

239 

238 

288 

277 

252 

309 

251 

241 

248 

245 
272 
269 
299 

255 

358 

264 
267 
258 

265 

296 

246 

305 
























682 


INDEX 


Test No. Page 

Motors, Testing of Asynchronous A. C. Induction . . . 260 

-Testing of Continuous and A.0. Electro- . . . 232 

103 --AVith Variable Rotor Circuit Resistance, Relation between 

Efficiency, Slip, Torque, Load in an Induction . . 294 

77 Motors and Dynamos, Analysis of Internal Loss of Power in . 202 

49 -and Dynamos, Measurement of the Insulation Resistance of 129 

Multicellular Electrostatic Voltmeter, Kelvin’s . . . 563 

100 Multi-phase A C. Electro-Motors, Efficiency and B.H.P. of . 272 

146 & 149 -A C. Rotatory Converters, Measurement of Efficiency 

of . . . . . . . 433 & 439 

143 -A.C. Static Tran.sformers, Measurement of Efficiency of .424 

50 Murray’s Loop Test for faults in Cables . . . .130 

120 Mutual Inductive Effects due to Relative Positions of 2 Coiled 

Circuits ....... 343 

N 

Nalder Low Resistance Measurer . . . . .521 

-Potentiometer ...... 507 

Nickel-Chrome Resistance Material, Table for . . .647 

144 Nodon Valve Rectifier, Efficiency of a . . . . 427 

Non-Inductive Wattmeter, Conditions for, and Object of a . 404 

147 No- Load “ Open Circuit ” Characteristic of Rotary Converter . 438 

136 “ Open Circuit ” Characteristic of Static Transformer . 404 

-“ Open Circuit ” Loss in an A.C. Static Transformer . 402 

97 -“ Open-Circuit ” Test on Induction Motor . . . 264 

98 -Short Circuit Test on Induction Motor . . . 267 

101 -Test for Efficiency of Induction Motor (Heyland Method) . 277 

Notation for Points in Curve Plotting ... .3 

Numbers and Constants, Useful ..... 654 

O 

Ohmmetcr, Description of the principle of the Evershed . . 539 

Open Circuit Losses in Transformers .... 402 

67 -Circuit Characteristic of an Alternator . . . 169 

147 --Circuit (No Load) Characteristic of Rotary Converter . 438 

97 -Circuit (No Load) Test of Induction Motor . . . 264 

156 Oscillograph, “ Wave-forms ” by Duddell. . . , 451 

P 

Parr’s Direct-Reading Dynamometer Measuring Instruments . 572 

155 Periodic E.M.F. and Current Curves of an Alternator (Ballistic 

Method) ....... 450 

154 -E.M.F. and Current Curves of an Alternator (Electrostatic 

Method) . . . . . . .446 

121 Permeability by the Permeameter, Measurement of Magnetic . 345 

122 -by Hopkinson’s Permeameter, Measurement of Magnetic . 347 

121 Permeameter, Measurement of Magnetic Permeability by the . 345 

122 -Measurement of Magnetic Permeability by Hopkinson’s . 347 

-The ... ..... 615 

112 Phase Relations between A^oltages and between Voltage Current 

in Circuits of Capacity in Series with Ohmic Resistance . 323 






















INDEX 


683 


116 


Test No. 

115 Phase Relations between Main and Branch Currents in Circuits 
having Resistance in Parallel with Self-Ind. or Capacity 

-Relations between the Currents in Circuits having Capacity 

and Self-Ind. in Parallel, Variation of Impedance and 
Photometer Bench, Table giving Ratio of Squares of Distances for a 

-Screens (Bunsen’s, Jolly’s and Lummer Brodhun’s) 

-Trotter’s Direct-Reading Bar. 

-Trotter’s Illumination 

Photometric Cradle, Arc Lamp 

■-Standard of Light, the Methven Screen 

Photometry of Electric Arc Lamps 

-of Electric Glow Lamps 

-Use of Coloured Glass in . 

Planimeter (Amsler’s), Instructions for getting area of Indicator 
Diagram ..... 

-(Amsler’s), Instructions for working the 

Plotting Curves ..... 

Pohl’s Commutator. .... 

Polar Curves of Distribution of Potential around Commutators 

-Curves of Light Distribution from an Electric Arc Lamp 

-Curves of Light Distribution from an Electric Glow Lamp 

Polyphase A.C. circuits, Measurement of Power in 
Poole’s Electrical Method for Efficiency of D.C. Motors 
Portable Galvanometer, Sensitive . 

-Testing Set (Evershed’s), Description of a 

-Testing Set (Silvertown), Description of . 

Post Office form of Wheatstone Bridge, Measurement of Resistance 
by the ........ 

Potential around a Commutator, Polar Curves of Distribution of. 

-Difference Method of Measuring Low Resistance . 

-Difference Method of Measuring Low Resistance, Proof of 

Formula in . 

-round the Commutator of a Dynamo, General Remarks (and 

Thompson’s method) on Distribution of . . . 

round the Commutator of a Dynamo (Mordey’s method), 


133 


109 


134 


135 


90 


34 


35 


Page 

332 

333 
651 

592-5 
689 
590 
587 
595 
61-72 
50-60 
53 

530 
528 
1 

583 
200 
68 
56 
388-400 
. 258 

. 571 
539-546 
. 434 


75 


76 


39 


81 
200 
84 

492 

195 

199 

201 
510 
94 

514 

515 
507 
521 

Power absorbed in A.C. Circuits, Three-voltmeter Method of find¬ 
ing the Electrical . . . • • .379 

-Factor, Measurement of . . . 292, 316, 398 

-in Alternating Current Circuits, the Apparent . . 403 

-in Alternating Current Circuits, the True . . . 403 

_in Alternating Current Circuits, Three-ammeter Method of 

finding the Electrical . . . . . . 383 

_in 2-phase A.C. Circuits, Measurement of . . 393 & 395 

_in 3-phase A.C. Circuits, Measurement of . . 388 & 395 

-transmitted by Belts ...••• 623 

Practical and C.G.S. units, Table showing Relation between . 643 


Distribution of 

-round the Commutator of a Dynamo (Mordey-Swinburne), 

Distribution of ...... 

Potentiometer, Crompton’s ..... 

Method of Comparing Resistances by . . . 

Method of “ Setting ” Crompton’s . . 

Precaution in using, and Sources of Error in . . . 

The N. C. S. 

Volt Box 

































684 


INDEX 


Test No. Page 

Preparation of Iron Samples for Magnetic Hysteresis Test . 350 

Price’s Guard-wire in Tests on Insulation Eesis+ance of Cables . 100 

Prony Brakes 234 & 633 


126 


118 


119 


144 

145 


138 

139 


34 

39 

44 

108 


20 


42 
49 

47 

43 

45 

46 

48 


40 

37 

36 


R 

Reactance, Impedance and Self-Ind, of Alternators, Motors, Trans¬ 
formers, etc., by Alternating Currents . . • 358 

-Impedance and Self-Ir.d. with Position of Movable Core in 

Solenoidal Choker, Variation of ... • 339 

--Impedance, Self-Iud., Current and Power, Effect of Bength of 

Air Gap in a Closed Magnetic Circuit on . . . 341 

-of a Cable with Alternating Currents . . . 373 

Reciprocals of Numbers, Tables of .... 663 

Rectifier, Efficiency of a Nodon Valve .... 427 

-Efficiency of a Rotary ..... 431 

Reflecting Galvanometers, Deviation of deflection from direct 

proportionality . . . . . .490 

Regulation ofStatic Transformers from “open”& “shortcircuit” 

tests. Deduction of . . . . . . 410 

-(by Differential method) Determination of . . . 410 

-(in Single conversion method) .... 412 

Reostene Resistance Material, Table giving Rise of Temperature 

for Different Currents in . . . . .648 

Resistance, Anti-Inductive, for Kelvin Standard Balances . 553 

-Approximate Test for a very Low .... 524 

-by Interpolation, Calculation of Exact . . ,83 

— by the Post Office Wheatstone Bridge, Measurement of . 81 

— by the Wheatstone Bridge, Proof of Formula giving . 492 

— Comparison of High, Medium, and Low (Potentiometer 

Method) ....... 94 

— (Conductor), Measurement by Evershed’s Bridge-Megger . 117 

— Determination of the Non-inductiveness of a . .314 

— Fixed Standard Low ...... 604 

— General remarks on Insulation . , , 93 & 109 

— heated by a Current, Measurement of a . . .46 

-— (High) by the Direct Deflection Method, Solution of I ti Ter¬ 
ences for ...... . 493 


Incandescent Lamp Box 

(Insulation) by Silvertown Portable Testing Set 
(Insulation) of Dynamos and Motors, etc. 
(Insulation) of Electrical Mains and Installations 
(Insulation) of Faulty Telegraph Insulators . 
(Insulation) of Installations, Cables, Machines, 


etc., 


. ‘598 
. 105 

. 129 

98 cS: 109 
. 123 

by 


Evershed’s Megger & Bridge-Megger Portable Testing Sets 113-110 

- (Insulation) of Installations, etc.. Measurement while working 119 

-(Insulation) of Installations, etc., Measurement while 
working . . . . . . .121 

- (Insulation) of Storage Batteries .... 127 

- Low) by the Potential Difference Method, Solution of 

Inferences for. . . . . . . 492 

- (IjOw), Measurement by Nalder Measurer . . .97 

- (Low), Measurement by Simple Potentiometer Method . 88 

- (Low), ]\Ieasurement by Voltmeter and Ammeter Method . 86 

- Material Table for Eureka , ... 646 
































INDEX 


685 


Test No. Page 

Kesistance Material Table for Manganin .... 649 

-Material Table for Nickel Chrome . . . ,647 

-Material Table for Eeostene . . . . , 648 

-Measurer, Description of a Low .... 521 

-of a Dynamo Circuit, Graphical way of finding the . . 154 

41 —^— of Cables (Direct Deflection Method), Measurement of the 

Insulation , ..... 99 

35 -of Dynamo Armatures, Electric Light Cables, etc.. Measure¬ 
ment of the ....... 84 

-of Dynamos and Motors, Proof of Formula giving Insulation 496 

-of Electric Light Installations while working. Proof of 

Formula giving the Insulation 
-of Liquids, Specific 

81-of Magnet Coils, Rise of Temperature and increase in 


38 


32 


of Metallic Conductors by Silvertown Portable Testing Set 
of Pure Metal and Alloys, Table of Specific . 
of Secondary Cells, Measurement of the Internal 
of Secondary Cells, Proof of Formula for the Internal 
of Single & Polyphase Windings, Measurement of . 
of Storage Battery, Proof of Formula giving Insulation 
Siemens Bridge for Low .... 

Test by Siemens Low Resistance Bridge 


494 
645 
216 
90 
644 
. 75 

. 491 

. 263 

. 496 

. 525 
. 525 

. 3.30 

. 334 

. 190 

. 585 

. 619 

. 596 

. 600 
600-604 
. 606 
.607 

81 Rise of temperature and increase in Resistance of Magnet Coils . 216 

146 & 149 Rotary Converters, Efficiency of Multi-phase . 433 & 439 

^47 ___ Converters, No-Load Characteristic of 

148-152 -Converters, Other Tests on 


Resonance (Pressure) in Series Circuits 

-(Current) in Parallel Circuits 

Retardation Method of Measuring Iron Loss in Alternators 
Reversing Key, Simple 
Revolving Contact Maker . 

Rheostat, Adjustable Carbon 

-Adjustable 

-Continuously variable 

-Stand coil (step by step) 

Three-phase liquid 


145 

52 

128 

127 


Converters, Synchronizing of. 
Recti6er, Efficiency of a 


. 438 

439-443 

. 441 


Rotational Speed, Measurement of, by Stroboscopic Fork . 

Rotor of an A.C, Motor, Definition of the . 

Rowland’s Method of Comparing Self-Induction by Alternating 

Currents . • • ai.. ‘ 

_Method of Measuring Self-Induction absolutely by Alter- 

nating Currents ....«• 


431 

135 

270 

365 

362 


S 

Scarf-Joint in Bare Wires . . . 

Screens (Bunsen’s, Jolly’s and Lummer-Brodhun s). Photometer 592-5 

-(Methven’s) Photometric Standard of Light_ • • • 

48 Secondary Batteries, Measurement of the Insulation Resistance of 127 

33 Cells, Measurement of the Efficiency and Storage Capacity of 7b 

32 Cells! Measurement of the Internal Resistance of . * q? 

_Cells Proof of Formula for the Internal Resistance of .491 

































686 


INDEX 


Test No. 

127 Self-Induction, Absolute Measurement (by Rowland’s A.C. 

Method) of ... . ... 

(by Impedance Method), Measurement, using Single-phase 


126 

114 

128 

118 

119 

126 

129 

127 

57 

55 

56 
53 


124 

58 

59 

89 

90 

69 

98 

137 

60 

61 

53 

92 


A.C.s of 

- Capacity, and Ohmic Resistance in Circuits, Effects of 
Frequency with ...... 

- Comparison (by Alternating Current) of two Coefficients of . 

- Impedance and Reactance with Position of Movable Core in 

Solenoidal Choker, Variation of ... . 

- (Impedance Method using Single-phase A.C.s), Proof of 

Formula ..... . . 

- Impedance, Reactance, Current and Power, Effect of Length 
of Air Gap in a Closed Magnetic Circuit on . 

- Impedance, Reactance of Alternators, Motors and Trans¬ 
formers, etc., by Alternating Currents 

- in Series and Parallel, Determination of Laws of Com¬ 
bination of ...... . 

- in Series and Parallel, Proof of Laws of Combination 
Measurement (by Rowland’s A.C. Method) of 


Separately Excited Dynamo (at Constant Speed), Relation between 
External and Exciting Currents .... 
•Excited Dynamo, Determination of the “Characteristic” 

of a. . . . . . . . 

Excited Dynamo, Internal Characteristic or Curve of Mag¬ 


netization of 

Excited Dynamo, Relation between Speed and E.M.F. in 


Separation of Internal Losses in an Alternator 

-of Internal Losses in Dynamos and Motors, by Housman’s 

Method ....... 

-of Internal Losses in Dynamos and Motors, by Kapp’s 

Method ....... 

of Iron Losses by A.C. Frequency Method 


Series Wound Dynamo, Critical Resistance at a given Speed in a. 

- Wound Dynamo, Determination of External Characteristic 

of a. . . . . . . . 

- Wound Dynamo, Determination of Total Characteristic of a 

- Wound Dynamo, Internal Characteristic or Curve of Mag¬ 
netization of a . 

Wound Electro-Motors, Efficiency and B.H.P. of Direct 


Current 

-Wound Tramway Motors, Efficiency and B.H.P. of 500 Volt 

D. C. . . . . . . j . 

Short-circuit Characteristic of an Alternator 
and No-Load Test of Induction Motor 
Test for Copper Losses in Transformers 


Shunt Wound Dynamo, Critical Resistance of a . . , 

■ Wound Dynamo, Determination of Characteristic of a 

• Wound Dynamo, Graphical Deduction of Total Character¬ 
istic in. 

• Wound Dynamo, Internal Characteristic or Curve of Mag¬ 
netization of a . 

• Wound Dynamo, Relation between Speed and E.M.F. in 
Wound Electro-Motors, Efficiency and B.H.P. of Direct 


Current 

Siemens Dynamometer-Wattmeter 


Page 

363 

358 

328 

365 

339 

498 

341 

358 

366 
498 
362 

149 

147 

148 
140 
189 

204 

203 

352 

154 

150 
152 

155 

241 

245 

173 

267 

407 

159 

155 

157 

159 

140 

248 

680 






























INDEX 


687 


Test No. 

Siemens Electro-Dynamometer , 

-H.P. Transmission Dynamometer 

-Low Kesistance Bridge 

Torsional Spring Control Voltmeter 


42 

38 


99 


142 


140 


139 


141 


Silvertown Portable Testing Set 

Portable Testing Set, Insulation Resistance by 
-Portable Testing Set, Measurement of Metallic Resistance 

by lEe . . . . . 

Portable Testing Set, Table showing best w'ay to work the . 
Sine and other Curves of E.M.F, Waves of a Dynamo 
Single-phase A.C. Electro-Motors, Measurement of Efiiciency and 

B. U. P, 

-Phase A.C. Static Transformers, Efficiency by Blakesley’s 

Method of ...... . 

-Phase A.C. Static Transformers, Efficiency by Double Con¬ 
version of ...... . 

-Phase A.C. Static Transformers, Efficiency by Single Con¬ 
version of ...... . 

-Phase A.C. Static Transformers, Efficiency by Sumpner’s 

Method of ...... . 


Page 

577 

623 

525 

575 

534 

105 

90 

92 

197 

269 

422 

414 

412 


139-142 
105 — 


Phase A.C. Static Transformers, Efficiency of 


Phase Commutator Motors, Efficiency and B.H.P. of 


118 


417 
412-424 
. 299 

Slanting T-Joint in Electric Light Cable .... 486 
Slip in an A. C. Motor, Definition and Measurement of Magnetic 260-262 

637 
474 


65 

53 

52 

51 

88 

66 
87 
86 


Soames’ Motor Testing Brake 

Soldering Irons for Cable Jointing. . . . . 

Solenoidal Choker, Variation of Impedance, Reactance and Self- 
Induction with Position of Movable Core in , 

Specific Gravities, Table of. 

-- Resistances of Liquids, Table of . 

-Resistances of Metals and Alloys, Table of . 

Speed and E.M.F. at which a Dynamo truly Compounds, Deter¬ 
mination of . 

-and E.M.F. in Direct-Current Dynamos, Relation between . 

-by Stroboscopic Fork, Measurement of . . . 

-Frequency, and Slip by the Stroboscopic Method 

-- Indicators, Calibration of . 

-Voltage and Current with Brush Position on Commutator 

of D.C. Motors, Variation of . 

-with E.M.F. in Alternators (at Constant Excitation), 

Variation of . 

-with Excitation on Armature of D.C. Motors (at Constant 


27 


Voltage) ....... 

-with Voltage on Armature of D.C. Motors (at Constant 

Excitation) ....... 

Speed-Torque Cures of an Induction Motor, General Form of 
Speeds of Target with Stroboscopic Fork, Table of • . 

Spherical Candle Power of an Electric Arc Lamp . 

-Candle Power of an Electric Glow Lamp 

-C.P. and Efficiency, Determination of the effect of various 

Carbons on . . • 

Spring tapping key, two-way high insulation 

-Transmission Dynamometer, Expression for amount of 

Coiling of Spring ...... 

- 1.1 Transmission Dynamometer, Stroud . . « • 


339 

644 

645 
644 

167 
140 
135 
262 
134 

239 

168 

238 

236 

295 

140 

70 

56 

70 

586 

632 

626 




























688 


INDEX 


Test No. 


Page 


91 

104 

138 


159 

52 

141 

102 

76 

95 

82 


106 

107 


Springs for Thompson’s Steam-Engine Indicator, Table of . 534 

Square Roots for Kelvin Balances, Tables of doubled . . 665 

Squares of Numbers, Table of ..... 661 

Stand Coil Rheostat ..... . 606 

Standard Cell, Preparation of the Clark .... 504 

Standard Kelvin Balance . . . . . .546 

Standardization of Electrical Measuring Instiurneiits, General 

remarks on the ...... 4 


Starting Torque and Cnrrent in D.C. Mutors, Relation between . 246 

-Torque, Voltage, Current, and the Rotor Circuit Resistance 

of an Induction Motor, Relation between . , . 296 

Static Transformers (from Differential Method) Determination of 

Regulation of . . . . . . . 410 

-Transformers (from “open” and Short Circuit Tests) De¬ 
duction of Regulation of ..... 410 

-Transformers, Fundamental Considerations relating to A.C. 400 

Stator of an A. C. Motor ...... 270 

Steam-Engine Generating Sets, the Commercial Efliciency of . 468 

Stray Power in Dynamos and Motors . . . 220, 255 

Stroboscopic Fork, Measurement of Rotational Speed by . .135 

--Target, the . . . . . . . 138 

Sumpner’s Method of Measuring the Efficiency of Single-phase 

Static Transformers . . . . . .417 

Sumpner-Weekes Method of Testing Induction Motors . . 288 

Swinburne-Mordey Method of finding Distribution of Potential 

round Commutators . . . . . .201 


Swinburne’s Electrical Method of Measuring Efficiency and B.H.P. 

of Electro-Motors . . . . . .255 

-Electrical Method of Measuring Efficiency of D.C. Dynamos 219 

Switch, Change-over and Reversing .... 582 

-Two-way sliding ...... 587 

Synchronizing Rotary Converter . . , . .441 

Synchronous Alternating Current Motors . . . 269 & 305 

•-A.C. Motor, Variation of Excitation and Armature Current 


and the “V” with other Curves of a . . . . 305 

• A.C. Motors, Efficiency and B.H.P., etc., of . . 309 

• A.C. Synchronous Motors, etc.,by Lamps and “Synchroscope” 306 

■ Speed, Definition with Induction Motors of. • , 260 


T 


51 

47 

110 


81 


Tachometers, Calibration of ... , 134-140 

Target, the Stroboscopic ...... 138 

Telegraph Insulators, Measurement of the Insulation Resistance 

of faulty ... . . . .123 

Temperature and Frequency in Circuits having Ohmic Resistance 

and Self-Induction only. Relation between . . . 319 

-Coefficient of Metals and Alloys, Table of . . . 644 

-Coefficient of variation of E.M.F. in Clark’s and Weston 


Colls ...... 

- Rise and Increase in Resistance of Magnet Coils 


Testing Sets, the Evershed 
-the Silvertown 


Thompson Steam-Engine Indicator 
Thompson’s Method of Sampling the E.M.F. of Armature Coils . 198 


13, 17 & 643 
. 216 
539-546 
. 534 

531 
















INDEX 


689 


142 

135 


16 

133 


91 

103 


of Measuring 


the Electrical Power in 


Page 


Test No. 

134 Three-Ammeter Method 

A.C. Circuits.. 

Three-Dynamometer Method of Measuring Transformer Efficiency 422 
Three-phase A.C. Circuits, Measurement of Electrical Power 

in. . . . . . . . 388 & 395 

-Liquid Rheostat ...... 607 

Three-Voltmeter Method of Calibrating a Wattmeter ! *. 38 

Method of Measuring the Electrical Power absorbed in aii 
A.C. Inductive Circuit ..... 

-Method of measuring power in A.C. Circuits, Proof of 

Formula .... 


T-Joint between two Electric Light Cables on the slant 

-between tivo Gutta-percha-covered Wires 

-between two Lead-covered Electric Light Leads 

-between two single-wire Electric Light Leads 

-between two 7-stranded Electric Light I-eads 

-between two 19-stranded Electric Light Leads 

between 37-stranded Electric Light Leads 


379 


Torque and Current of D.C. Motors, Relation between Starting . 

-Efficiency, Slip, Load, etc., in an Induction Motor with 

Variable Rotor Circuit Resistance .... 

-exerted by a Motor, Expression for . , 

-exerted by Series Motors, Expression for . 

-speed Curves of Induction Motor, General form of . 

“ Total Characteristic” of an Alternator (analytically) 

-Characteristic” of an Alternator (graphically) 

■-Characteristic ” of Compound Dynamo (Long Shunt), 

Graphical deduction of the ..... 

•-Characteristic ” of Compound Dynamo (Short Shunt), 

Graphical deduction of the ..... 

-Characteristic ” of Magneto Dynamo, Graphical deduction 

of the . ....... 

-Characteristic ” of Series Dynamo, Graphical deduction of 

the ........ 

Characteristic ” of Shunt Dynamo, Graphical deduction of the 


501 

486 

484 

484 

477 

481 

482 

483 
246 

294 
246 
241 

295 

179 

180 

162 

164 

145 

152 

157 

609 

245 

265 

410 

407 


Tracer, Ewing’s Magnetic Curve 

90 Tram and Piailway Motors, B.H.P. and Efficiency of 500 Volt 
Direct Current....... 

Transformation Ratio in Induction Motors 
138 Transformers (by DilTerential Method) Determination of Regu¬ 
lation of ...... . 

137 -(by Short Circuit Test) Copper Losses in . 

142 -Efficiency (by Blakesley’s 3-dynamometer Method) of Alter¬ 
nating Current ...... 

240 --Efficiency (by double conversion) of Alternating Current 

2 39 -Efficiency (by single conversion) of Alternating Current 

242 -Efficiency (by Sumpner’s Method) of Alternating Current . 

243 -Efficiency of Multi-phase Alternating Current 

139-142 -Efficiency of Single-idiase Alternating Current Static 

-(from Open and Short Circuit Tests) Deduction of 

lation of . ... . • • 410 

-Fundamental Considerations relating to A.C Static . 400 

126 _Impedance, Reactance andSelf-Ind. by Alternating Currents 358 

85 Trajismission Dynamometer, Measurement of Efficiency of a 

Generator by a . . . • • • 230 


422 
. 414 

. 412 

. 417 
. 424 

412-424 


Regu- 



























G90 


INDEX 


Test No. rage 

Transmission Dynamometer, Siemens H.P. . . • 623 

-Dynamometer, Spring ..... 625 

-of Power by belts ...... 623 

Trotter’s Direct reading Bar Photometer .... 589 

-Illumination Photometer ..... 590 

Twist Joint between Gutta-percha-covered Wires . . .484 

-Joint between Lead-covered Cable .... 483 

-Joint between Single Core Electric Light Wire . .476 

-Joint between 7-stranded Electric Light Cable . .479 

-Joint between 19-stranded Electric Light Cable . . 482 

-Joint between 37-stranded Electric Light Cable . . 483 

Two-phase A.C. Circuits, Measurement of Electrical Power in 393 & 395 
Two-way high insulating spring tapping key . 586 

-ordinary simple sliding key . . . . .587 


a 

Units, Table showing relation between practical and C.G.S. . 643 

Useful numbers and constants ..... 645 

V 

106 “ V ” Curves of Synchronous Motors, Determination of . . 305 

88 Voltage, Current, and Speed with Brush Position on Commutator 

ofD.C. Motors, Variation of ..... 239 

-“drop ” of an Alternator with Load and Power Factor . 174 

-ratio of conversion in Multi-phase Rotaries . . . 437 

148 -ratio with excitation and speed in Rotaries, Variation of . 439 

86 -with Speed ot Armature ofD.C. Motors (at Constant Exci¬ 
tation), Variation of . . . . . . 236 

Voltameter, Useful details for obtaining a good copper . . 14 

Volt Box for the Potentiometer . . . . .521 

11 Voltmeter calibration by comparison with a Crompton Potentio¬ 

meter ........ 23 

10 -calibration by comparison with a Kelvin Centi-Ampere 

Balance ...... .22 

9 --calibration by comparison with a Kelvin Composite Balance 21 

8 -calibration by comparison with a Standard D’Arsonval 

Voltmeter ....... 18 

7 -calibration by PoggendorfF’s Method . . . .16 

12 -calibration of a High Tension Alternating Current . . 26 

13 Voltmeters, Complete test for various sources of Error in Direct 

and Alternating Current . . . . .29 

-Electrostatic ....... 562 

-Kelvin Multicellular Electrostatic .... 563 

-Siemens Torsional Spring Control .... 575 

W 

78 Waste Field of Dynamos, Determination of relative distribution of 207 
117 Wattless or Idle Current and Load Current, Determination of 

the ...... . 336 & 390 

14 Wattmeter calibration by comparison with a Kelvin Composite 

Balance . . . . , .34 

























INDEX 


691 


17 

16 


Test No. Page 

15 Wattmeter calibration by comparison with 

37 
41 

38 
35 

. 399 

. 580 
. 404 

4 

446-451 

Weekes-Sumpner Method of testing 3-Phase Induction Motors . 288 


Standard Ammeter 

and Voltmeter. 

- calibration of a High Tension, by application of Ohm’s Law 

- calibration with Alternating Currents (3 Voltmeter Method) 

-Constant of a . 

-Correcting Factor of a 

-Siemens Dynamometer 

-the conditions for and object of a 

156 Wave Forms by the Duddell Oscillograph 

154-5-by Joubert Contact Maker . 

102 


Weston (Cadmium) Standard Cell, E.M.F. with temperature of 13 & 643 
34 Wheatstone Bridge, Measurement of Kesistance by the Post Office 

form of ....... 81 


-Bridge, Method of Measuring Resistance, Proof of Formula 

for .. ...... 

Windings, Resistance of Single and Polyphase 
“ Wire-drawing ” in Steam-Engines . . . . 

-Gauges, Comparison of Different . . ^ . 


492 

263 

471 

650 
























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